Snow chance early Tuesday, but big uncertainty
Despite all of this week's warmth, and the increasing possibility President's day may be pretty warm itself, we're carefully monitoring the potential for crashing temperatures Monday night while a storm system approaches from the west and southwest. This system may bring rain that changes to snow by Tuesday morning, potentially impacting the back-to-work rush hour. This is a very complicated set up which we will be providing updates about through the holiday weekend.
Wes Junker provides an excellent technical analysis below - which is worth a read to understand why this forecast is so challenging and fraught with uncertainty. - Jason Samenow
Keep reading for Wes Junker's in-depth analysis...
Monday I concluded that the overall weather pattern was not that favorable for snow but could produce surprises (i.e. a snowstorm) if enough blocking developed (i.e. a negative Arctic Oscillation) in the North Atlantic.
At that time, the European and Canadian models were alone in one camp suggesting that an initial low would track to our north on Monday which would help pull cold air back to the south over our region prior to the approach of a second wave on the front that could produce snow. Since then, the GFS has trended toward that European model idea and has shown a low of varying intensities tracking to our south on Tuesday advertising the potential for snow. In fact some of its forecasts from yesterday were quite aggressive about the potential.
The models continue to suggest potential for a little rain on Sunday night or Monday morning as the first surface low passes by. The low passing to our north would provide the area with southerly winds putting us in the warm sector south of the front. Temperatures during the day Monday based on last night's GFS could rise well up into the 60s.
The forecast difficulty starts once that low passes far enough away to our northeast, the front sinks south, and a weaker second surface low develops on the front and approaches us from the west. How far south of our area this second low tracks will determine whether we get mostly rain, rain to snow, or get little or no precipitation.
The big problem for snow lovers is that the second low has to track far enough south for the cold air to settle southward firmly into the Washington area but not so far south that the precipitation misses us. Right now the models continue to vacillate between solutions with the Euro forecasting mostly rain but possibly ending as snow while the 1AM run of the GFS tracked the low so far south that we would get snow but only about an inch of snow because the bulk of the precipitation passes to our south. Today's GFS run has shifted the low back to the north making it wetter but also warmer at least during the first half of the storm. Any of these scenarios remain possible.
Examination of two GFS runs last night illustrates how small differences in two features (described and illustrated below) and their interaction between each other can subtly change the storm track leading to significant differences in the storm's precipitation field.
Note that on the chart above there is a vortex (the oval shaped lines near Maine) in both panels as well as a dip in the lines over the Midwest with an x or x's in the vicinity of Kentucky. This trough (dip in the lines) provides the upper support for the surface wave that is forecast to track south of the region. On the charts I've drawn an arrow pointing to where the model runs are showing confluence (where the lines start coming together as you move east to west).
Why care about confluence? The surface pressure usually rises in regions having upper level confluence. If you shift the confluence southeast from the New York-Pennsylvania border (left panel) to closer to the Mason-Dixon line (right panel) then the surface low will also be forced farther to the south as will the precipitation shield.
Notice on the two charts that line labeled 540 on the left panel extends farther to the south and west than on the right hand panel. That dip in the lines changes where the confluence is located and also produces a stronger upper level support for the surface low.
If you move the vortex near Maine a little to the north and northeast, the approaching trough probably would stay a little farther north which would delay how quickly the cold air come southward into our area in advance of the wave as simulated in last night's European solution and this morning's GFS.
In fact, this morning's GFS has jumped back northward with the storm track taking a low to West Virginia by 7PM Monday evening then tracking it to Petersburg, Virginia by 1AM Tuesday morning. The temperatures in the lowest 500 feet do not fall below freezing until the bulk of the precipitation has fallen. However, it does suggest that there could be a transition of sleet to snow across the area with the northern areas changing earlier than the southern ones. And taken literally, it would suggest the cold air and snow to coincide with Tuesday morning's rush hour with at least a light accumulation.
The run to run changes of the GFS solutions are a signal that there is tremendous uncertainty in the forecast. A plume diagram above that shows the precipitation from all the GEFS ensemble members clearly illustrates how much uncertainty there is about how much and what types of precipitation might fall. Remember that ensemble members have the same physics as the GFS but the initial conditions have been tweaked to get an idea of the predictability of the system.
The 06Z ensemble members are all over the place. Precipitation type is color coded with the color code shown on the left hand side of the figure. Most start the precipitation as rain and then change it to snow but some show only light snow at the end while some show a significant portion of the precipitation falling as snow.
So what can we say about the system? A surface low is likely to track across or to the south of the area. It is likely to produce precipitation and some of it will probably be in the form of rain and there is a chance of snow. It's too early to estimate accumulation potential but the warm surface temperatures on Monday are a big negative that will have to be overcome.
Nevertheless, we need to watch this very carefully as there is a chance of snow and falling temperatures coinciding with Tuesday morning's around rush hour after the long holiday weekend.
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