Light Snows tomorrow night – late week system looks weaker

We’re watching a weak and unfocused system which may provide some rain and light accumulating snowfall midweek, while according to the latest guidance, prospects for a late week wintry system across the southern part of the district fade.

Outlook

Wednesday…Partly sunny west..mostly sunny east.  Highs in the mid to upper 30s.

Wednesday Night & Thursday…Snow developing across the region in the evening west and after midnight east.  Light accumulations of an inch or two.   Temperatures in the upper 20s to around 30 Wednesday night. Light snow ends as drizzle on Thursday as temperatures rise into the mid 30s.

Friday through Sunday…Gulf coast system will brush southern areas with the possibility of a light snowfall.  This does not look like a major event at this time, but forecasts need to be monitored for changes. More certainty after tomorrow when the system moves onshore in the west. Generally partly sunny to mostly cloudy elsewhere.

Christmas Week…Slightly above average temperatures to start.. highs in the low to mid 40s Monday and Tuesday with a chance for rain. Possibly trending cooler Christmas Eve/Day. Unsettled weather possible and the pattern is changeable.

Weak midweek system to give some rain/snow potential:

A weak and disorganized system aloft will give the area the potential for some light accumulating snows from Wednesday evening through Thursday morning.  This is not associated with any significant surface weather system as can be seen from the chart below:

2014-12-16-1513-WPC00Z1216F

This type of situation poses several challenges:

1) How much moisture (potential rain or snow) will go into removing the dry lower layers of the atmosphere.

2) How will the several weak areas of “spin” in the atmosphere interact with each other?

3) Where will the best lift in the atmosphere be located?

This will be one of those situations where localized areas see a couple of inches and others just flurries.  That being said, here is the NWS snowfall forecast chart:

2014-12-16-1513-EDD72HSNWACC

That represents a good early estimation of the situation.

Prospects for late week/weekend snows are fading in the south

The operational weather prediction models have really continued to downplay the late week storm system, not only for our area but for the eastern seaboard.  As we discussed yesterday, a major issue with possible winter storms while out in the Pacific is the lack of a good sampling of the structure of the storm.   As of mid day, it is just offshore and should begin to be better sampled by the upper air network along the west coast as early as 6PM tonight and certainly by the next balloon release, which is 6AM Wednesday morning.  We’ll see if this significantly changes the weakening trend evident in all of the computer guidance.

2014-12-16-1242-EDDSATGLB

Lets take a look at the surface map forecast for 6AM Saturday which would be the “peak” of the wintry weather. Shown here are all four operational models available out through that time period.  (Click to make them more readable):

2014-12-16-1321-12ZECMSFC096 2014-12-16-1321-12ZGEMSFC096 2014-12-16-1321-12ZGFSSFC096 2014-12-16-1321-12ZGFXSFC096

All of the guidance now more or less has the low on or near the Louisiana gulf coast but only the GFS/GFSx models bring precipitation amounts of greater than 1/10″ of an inch liquid equivalent.  The European model brings around 1″ of snow to the Ohio River and SE IL, with 1/2″ as far north as SEMO/SW IL.  The GFS model brings 1/2″ to 1.5″ for SEMO/SW IL, less for STL and brings just rain as far north as Perryville MO to Carbondale IL. The Canadian model is similar.  The GFSx model brings totals of 1-1.5″ or less.

We’ll look to the ensembles as we’ve done for the past few days. In addition to the deterministic –or official run– of each computer model, there are 51 different “versions” (called members) of that run and one control version of the Euro model. They are all run using the same data at start time but are each run with slightly different equations. Doing this yields an average of all the versions in addition to the official version.

Take a look at how the different members all play out snowfall accumulations through Sunday morning.  Specfically, we’ll compare the number of members predicting at least 5″ of snow at 6 AM & 6PM yesterday compared to today:

Geographic area      6AM MON    6PM MON    6AM Today
Kansas City..........2 of 51....2 of 51....0 of 51  (2 fewer - now none expecting)
St. Louis...........13 of 51....8 of 51....3 of 51  (5 fewer)
SEMO/SW IL..........24 of 51...21 of 51....5 of 51  (16 fewer now only 10% of all)

That’s a pretty good indicator of a decreasing threat in a major event.  As mentioned above, we’ll see if this trend is maintained as we get a better sample of the storm system.  By this time tomorrow, we’ll also be able to look at the system within some of the higher resolution guidance. Early indications are this will be a light snow type event for the southern district with 1-2″ amounts at best.

Heading into Christmas Week…

The realignment of the upper air pattern is underway by that time and that makes this stretch lower confidence…especially late week.  It can be said with reasonable confidence that it will start out seasonable.. maybe a few degrees above average.  Highs in the 40s and lows near freezing or above look likely for Monday though Christmas Eve.  A weather system expected early in the week looks weak and mostly a rain-producer at this juncture.  A dry period would follow which may include Christmas Eve and Day with another system the weekend after Christmas. Near average temperatures–30s/40s for highs–20s for lows–are currently expected.  At least one model though does trend colder…

2014-12-16-1513-12ZGFXMKCGRP

Other models hold off until Dec 26/27.  Timing is an issue at this point, however and with the upper air pattern in transition, changes to the speed or track of next week’s systems could bring significant changes…so stay tuned.

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