Not much change in the thinking. A quick-moving storm will produce about 6 hours of snow tomorrow starting in the early afternoon most likely. I expect 1-2 inches but depending on where small-scale bands set up, more is possible in some areas, less as you go north.
The "storm" should be out of here by nightfall.
The weekend will be cold again but temperatures begin to moderate by Monday and we could be near 60 again by Wednesday before the next cold air arrives.
Rain probable next week with an outside chance of snow as the cold air arrives Wednesday night. But the next real chance for snow shows up on the models for the February 2-4 timeframe. Plenty of time to watch that one.
Enjoy whatever snow falls tomorrow. I'll post an update if there's any change.
Thursday, January 24, 2013
Wednesday, January 23, 2013
NAM "Clown Map" For Friday
Hot off the presses, the "Clown Map" for Friday which shows forecast snowfall amounts. You can see that the forecast from this model is between 1 and 2 inches for the DC metro. The interesting (and somewhat sad, given the last two winters) is that this is actually an improvement over previous NAM runs.
Maybe we're headed in the right direction...
Maybe we're headed in the right direction...
One Shot at Light Snow Tonight, Another on Friday
At the moment, it doesn't look like any major storms are in our near future. The best chance we had of a moderate storm was Friday but at this point, the storm looks like it will be weaker and will not produce all that much precipitation.
Let's look at where we are - first with tonight's event.
A weak "clipper" system is poised to move over our area overnight - probably after midnight and into the hours around dawn. The GFS and the NAM both show this clipper and show a decent amount of energy (for a storm like this) passing over our region. We can expect a dusting to perhaps an inch and the enjoyment of watching snow fall if you're up late enough (or early enough). The track is far enough south that areas around Fredericksburg could see the most snow - but even there, an inch or so is about as good as it will probably get.
Because of the very cold airmass that is currently locked in our region, the snow ratios should be higher than the standard rule-of-thumb 10-1. So even though the amount of available moisture is small, the chances of snow accumulation are pretty good. I'd say in the DC metro area we stand a 60-70% chance of seeing around an inch of snow tonight.
If the snow should continue into the morning rush hour, expect the NWS to put out Winter Weather Advisories.
This is a fairly narrow band of energy (as you can see from the model output above) so if we don't see much of anything on the ground and just a few flurries flying, I wouldn't be that surprised.
On to Friday:
The models are slowly coming into agreement and the news for snow-lovers isn't that good. The phasing I discussed in my previous post seems unlikely to happen as the southern stream energy gets shunted south and sheared apart. That means the northern stream energy has to work with whatever moisture it has and since it's continental in origin, that's not much.
Also, the storm will be a Miller B type scenario. Miller B storms are characterized by a primary low moving over land that weakens and transfers energy to a secondary low - usually off the coast. Sometimes this is a really good thing. For example, the Snowmageddon storm was a Miller B.
But.
In that case the energy transfer happened earlier and the southern stream low pressure system was much stronger (and had its origins in the Gulf Coast). As the storm moved up the coast, it actually almost stopped to our east before proceeding up the coast because of blocking patterns.
In this case, however, the southern stream energy is weak and the energy transfer may happen right over our area. These are bad things. If the energy transfer happens directly over us, we could be caught in the middle - what meteorologists call a "snow hole". That would result in us seeing very little snow at all.
The models are still in a bit of disagreement as to the path and the amount of moisture available to the system. At this point, it's unlikely that the storm will be anything to get excited about but we'll see how the models look this afternoon.
For now, there's a pretty good chance of light snow on Friday that could maybe result in 1-3 inches of snow if we're lucky.
Let's look at where we are - first with tonight's event.
A weak "clipper" system is poised to move over our area overnight - probably after midnight and into the hours around dawn. The GFS and the NAM both show this clipper and show a decent amount of energy (for a storm like this) passing over our region. We can expect a dusting to perhaps an inch and the enjoyment of watching snow fall if you're up late enough (or early enough). The track is far enough south that areas around Fredericksburg could see the most snow - but even there, an inch or so is about as good as it will probably get.
 |
| GFS model output showing light band of snow across the region overnight. Liquid equivalent precipitation forecasts are around 0.05 - 0.1 inches which translate to around an inch of snow. |
Because of the very cold airmass that is currently locked in our region, the snow ratios should be higher than the standard rule-of-thumb 10-1. So even though the amount of available moisture is small, the chances of snow accumulation are pretty good. I'd say in the DC metro area we stand a 60-70% chance of seeing around an inch of snow tonight.
If the snow should continue into the morning rush hour, expect the NWS to put out Winter Weather Advisories.
This is a fairly narrow band of energy (as you can see from the model output above) so if we don't see much of anything on the ground and just a few flurries flying, I wouldn't be that surprised.
On to Friday:
The models are slowly coming into agreement and the news for snow-lovers isn't that good. The phasing I discussed in my previous post seems unlikely to happen as the southern stream energy gets shunted south and sheared apart. That means the northern stream energy has to work with whatever moisture it has and since it's continental in origin, that's not much.
Also, the storm will be a Miller B type scenario. Miller B storms are characterized by a primary low moving over land that weakens and transfers energy to a secondary low - usually off the coast. Sometimes this is a really good thing. For example, the Snowmageddon storm was a Miller B.
But.
In that case the energy transfer happened earlier and the southern stream low pressure system was much stronger (and had its origins in the Gulf Coast). As the storm moved up the coast, it actually almost stopped to our east before proceeding up the coast because of blocking patterns.
 |
| Radar composite image from February 5-6, 2010 showing strong southern stream low receiving energy from a weaker low over the Tennessee Valley before moving up the coast. |
In this case, however, the southern stream energy is weak and the energy transfer may happen right over our area. These are bad things. If the energy transfer happens directly over us, we could be caught in the middle - what meteorologists call a "snow hole". That would result in us seeing very little snow at all.
The models are still in a bit of disagreement as to the path and the amount of moisture available to the system. At this point, it's unlikely that the storm will be anything to get excited about but we'll see how the models look this afternoon.
For now, there's a pretty good chance of light snow on Friday that could maybe result in 1-3 inches of snow if we're lucky.
Tuesday, January 22, 2013
Cold! Then Another Chance of Wintry Precipitation Friday
Cold is easier to forecast than snow, so let's start there.
It's cold. It's going to be cold - until Sunday, at least. The worst of the cold weather will be today through Friday. Highs in the 20's, lows in the teens and a noticeable wind which will make it feel even colder. Bundle up!
Now - let's have a look at Friday...
Most of the forecast models show a fast-moving low pressure system moving out of the southeast on Thursday night and making it here on Friday morning. That's about all they agree on however.
A few (GFS, Euro) show the storm tracking just to our south before reforming off the coast and moving out to sea. This would give us a relatively quick shot of precipitation but would keep it all snow.
Others (UKMET, NOGAPS) show the storm tracking just over us or just to our north. This would create an "overrunning" situation where warm air on the southeast side of the low slides over the cold air at the surface. Depending on the depth of the cold air, this can create the "mixed bag" of precipitation which starts and snow then changes to sleet or possibly freezing rain. I'd consider this the least likely scenario at this point.
The big questions as usual are:
An extra component to this situation is the cold air mass over our area. This cold will produce higher-than-normal snow ratios which means we can squeeze a bit more snow out of the same amount of precipitation.
Here is the GFS model showing precipitation overspreading the area on Friday:
You can see that the freezing line (blue) is quite a bit south of here which indicates a mostly snow event.
The storm remains a quick-mover and doesn't really get going until it's well off the coast. An event like this could produce 2-4 inches but what we want is a phase. In this scenario, the southern energy is sheared away which produces a weaker low.
The energy that will be producing this precipitation isn't even on the West Coast yet so there's plenty of time for things to change.
This is more of a "heads-up" that some snow is likely on Friday and it looks like there will be multiple parameters to consider before a solid forecast can be made. We'll see if there is any consensus from the models and what the energy looks like when it finally arrives.
Stay tuned.
It's cold. It's going to be cold - until Sunday, at least. The worst of the cold weather will be today through Friday. Highs in the 20's, lows in the teens and a noticeable wind which will make it feel even colder. Bundle up!
Now - let's have a look at Friday...
Most of the forecast models show a fast-moving low pressure system moving out of the southeast on Thursday night and making it here on Friday morning. That's about all they agree on however.
A few (GFS, Euro) show the storm tracking just to our south before reforming off the coast and moving out to sea. This would give us a relatively quick shot of precipitation but would keep it all snow.
Others (UKMET, NOGAPS) show the storm tracking just over us or just to our north. This would create an "overrunning" situation where warm air on the southeast side of the low slides over the cold air at the surface. Depending on the depth of the cold air, this can create the "mixed bag" of precipitation which starts and snow then changes to sleet or possibly freezing rain. I'd consider this the least likely scenario at this point.
The big questions as usual are:
- Storm track. Half the models are still forecasting a northward track.
- Phasing. There is a scenario where energy from the Pacific phases with the southern stream jet which then moves across our area. This scenario, by far, would produce more precipitation.
An extra component to this situation is the cold air mass over our area. This cold will produce higher-than-normal snow ratios which means we can squeeze a bit more snow out of the same amount of precipitation.
Here is the GFS model showing precipitation overspreading the area on Friday:
You can see that the freezing line (blue) is quite a bit south of here which indicates a mostly snow event.
The storm remains a quick-mover and doesn't really get going until it's well off the coast. An event like this could produce 2-4 inches but what we want is a phase. In this scenario, the southern energy is sheared away which produces a weaker low.
The energy that will be producing this precipitation isn't even on the West Coast yet so there's plenty of time for things to change.
This is more of a "heads-up" that some snow is likely on Friday and it looks like there will be multiple parameters to consider before a solid forecast can be made. We'll see if there is any consensus from the models and what the energy looks like when it finally arrives.
Stay tuned.
Thursday, January 17, 2013
Southward Track - No Significant Snow Expected
The southward track of the storm seems to be verifying and radar trends (and, frankly, the sky appearance) are not favorable for significant snowfall here in the DC area. I think some snow will fall, maybe a coating here and there on grassy surfaces - but nothing memorable.
This is another case of a storm which looked like nothing a few days ago, looked like something significant for two model runs yesterday and then beginning yesterday evening, started to look like nothing again. Inconsistencies in model runs like that within 24-36 hours of the start of the storm can cause forecasting havoc. This one sure did. When I stopped looking last night, the NWS was still forecasting 3-5 inches for the Fairfax area.
We're now 2 years into our snow drought. Cold air returns for the beginning to middle of next week but very little moisture will be passing by. I think there's a chance of snow showers on Inauguration Day but apart from that, looks like we'll have to wait a little longer for significant snow.
This is another case of a storm which looked like nothing a few days ago, looked like something significant for two model runs yesterday and then beginning yesterday evening, started to look like nothing again. Inconsistencies in model runs like that within 24-36 hours of the start of the storm can cause forecasting havoc. This one sure did. When I stopped looking last night, the NWS was still forecasting 3-5 inches for the Fairfax area.
We're now 2 years into our snow drought. Cold air returns for the beginning to middle of next week but very little moisture will be passing by. I think there's a chance of snow showers on Inauguration Day but apart from that, looks like we'll have to wait a little longer for significant snow.
Wednesday, January 16, 2013
More Questions Than Answers
The NAM says "no snow". The Euro says "no snow". The GFS and several short-term models insist on at least 2 inches of snow for the DC area.
At this point, there are more forecast questions than answers.
The official NWS forecast still calls for a storm with snow totals ranging from 2-6 inches across DC and points south. The snow numbers increase as you go further south with the "bullseye" across central and southwest Virginia. Locations to the north of DC are unlikely to see much, if any, snow at all if the current storm track holds.
As a forecaster, I would like to have more consensus from the models by now. I'd also like to NOT be right in the middle of the gradient of snow amounts - which means very small deviations in track could affect the outcome. To add complexity to an already-complex situation, small-scale events and "banding" could create localized areas of heavier snowfall just about anywhere from DC south.
Here is the Hydrometerological Prediction Center forecast map which shows the probability of seeing more than one inch of snow tomorrow until early evening. This is from a branch of NOAA which means they have considered all available model data from this evening when making this forecast:
You can see that the DC area is in the 60-70% range to see at least an inch of snow accumulation.
Change that to a parameter of at least 4 inches and the map changes dramatically:
This forecast has us in a very low probability - 5-10%.
Now this only applies until tomorrow early evening. If some of the models pan out, the snow could continue into the nighttime hours and more accumulation is possible. But based on this, I think a conservative forecast is in order with the usual caveats about storm track. Any shift to the north, and this becomes a bigger deal. Any shift to the south and we could be talking about another miss.
For now, I'll say 1-3 inches in the DC area, 2-4 points south of DC.
More model data is coming in and if there's any substantive change, I'll be sure to post it.
Those of you who love snow: hope for a northward shift which would get us in the "good stuff".
At this point, there are more forecast questions than answers.
The official NWS forecast still calls for a storm with snow totals ranging from 2-6 inches across DC and points south. The snow numbers increase as you go further south with the "bullseye" across central and southwest Virginia. Locations to the north of DC are unlikely to see much, if any, snow at all if the current storm track holds.
As a forecaster, I would like to have more consensus from the models by now. I'd also like to NOT be right in the middle of the gradient of snow amounts - which means very small deviations in track could affect the outcome. To add complexity to an already-complex situation, small-scale events and "banding" could create localized areas of heavier snowfall just about anywhere from DC south.
Here is the Hydrometerological Prediction Center forecast map which shows the probability of seeing more than one inch of snow tomorrow until early evening. This is from a branch of NOAA which means they have considered all available model data from this evening when making this forecast:
You can see that the DC area is in the 60-70% range to see at least an inch of snow accumulation.
Change that to a parameter of at least 4 inches and the map changes dramatically:
This forecast has us in a very low probability - 5-10%.
Now this only applies until tomorrow early evening. If some of the models pan out, the snow could continue into the nighttime hours and more accumulation is possible. But based on this, I think a conservative forecast is in order with the usual caveats about storm track. Any shift to the north, and this becomes a bigger deal. Any shift to the south and we could be talking about another miss.
For now, I'll say 1-3 inches in the DC area, 2-4 points south of DC.
More model data is coming in and if there's any substantive change, I'll be sure to post it.
Those of you who love snow: hope for a northward shift which would get us in the "good stuff".
NAM Comes in South
The 18Z NAM model run has come in further south with the storm which isn't good for snow lovers up here in the DC area. There's likely to be a pretty sharp gradient of total snowfall north of the center.
Let's hope this is just a one-run problem because the trend had been more favorable until now.
I will watch the other models as they come in.
Let's hope this is just a one-run problem because the trend had been more favorable until now.
I will watch the other models as they come in.
Winter Finally Arrives?
I'm watching a storm system that has become significantly more interesting in the past 24 hours. A low pressure system tracking from the southeast had previously been forecast to move quite far to the south, making any adverse effect on our region unlikely. In a "winter" full of near misses and no cold air, this didn't seem especially surprising.
However, in the last 24 hours, models have shifted their forecasts significantly northward. So much so, that we're approaching the "sweet spot" of the storm where cold air and moisture are co-located.
The result? Snow seems more likely now - and it could be a nice shot of snow during the daytime hours tomorrow.
The NWS office in Sterling has raised a Winter Storm Watch. The official forecast now is for 2-5 inches in the advisory area which covers most of the DC area. Because of the storm track, the totals actually increase to the south and areas around Fredericksburg could see more like 4-8 inches of snow.
If the storm track continues to shift north, some of that heavier snow could fall in the DC area.
Let's look at the maps:
Here's the GFS total forecast precipitation for the storm. This is from last night's model run...we're still waiting for the data from this morning.
You can see the precipitation is heaviest in central Virginia but this is a significant northward shift from previous runs of the GFS. That trend has continued and those darker areas which represent heavier snow are inching this way.
We should wait for the data from the morning model runs and the ones later this evening before making any solid forecasts. For now, let's say the chance of accumulating snow tomorrow is better than 75%, the chance of more than an inch is well over 50% and it's safe to assume 2 inches or so will fall in the DC area with more certainly possible depending on storm track...
I'll update when more data comes in.
For what it's worth, here's the current National Weather Service Snow Total Forecast Map. Our local NWS is well known for being conservative with snow storm forecasts so it's a bit of a surprise that they're really bullish about this storm. Maybe they've seen model data I haven't yet.
However, in the last 24 hours, models have shifted their forecasts significantly northward. So much so, that we're approaching the "sweet spot" of the storm where cold air and moisture are co-located.
The result? Snow seems more likely now - and it could be a nice shot of snow during the daytime hours tomorrow.
The NWS office in Sterling has raised a Winter Storm Watch. The official forecast now is for 2-5 inches in the advisory area which covers most of the DC area. Because of the storm track, the totals actually increase to the south and areas around Fredericksburg could see more like 4-8 inches of snow.
If the storm track continues to shift north, some of that heavier snow could fall in the DC area.
Let's look at the maps:
Here's the GFS total forecast precipitation for the storm. This is from last night's model run...we're still waiting for the data from this morning.
You can see the precipitation is heaviest in central Virginia but this is a significant northward shift from previous runs of the GFS. That trend has continued and those darker areas which represent heavier snow are inching this way.
We should wait for the data from the morning model runs and the ones later this evening before making any solid forecasts. For now, let's say the chance of accumulating snow tomorrow is better than 75%, the chance of more than an inch is well over 50% and it's safe to assume 2 inches or so will fall in the DC area with more certainly possible depending on storm track...
I'll update when more data comes in.
For what it's worth, here's the current National Weather Service Snow Total Forecast Map. Our local NWS is well known for being conservative with snow storm forecasts so it's a bit of a surprise that they're really bullish about this storm. Maybe they've seen model data I haven't yet.
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