A potent wintry night

[Monday December 22 update follows original entry.]

Looks like a potent night of weather coming up in the Midwest just a few days before the winter solstice.

Cold surface air is entrenched and difficult to dislodge (blue = temperatures below freezing as of mid-afternoon Thursday) ...

While a bit above the surface, warm & humid air (orange colors) wiill be surging in overnight, per this numerical forecast model depiction ...

[Source of this and subsequent images: wright-weather.com]

On the heels of strong "low-level jet" winds ...

That forcing will occur underneath a "coupled jet" pattern which is favorable for a further boost upward of the air motion and thus storminess and precipitation. A coupled jet can refer to two upper-level jet "streaks" such as below or a low-level jet coupled with an upper-level one (which will also be present tonight).

This is the sort of situation which in June would produce one of those nocturnal thunderstorm complexes along a warm front with continuous lightning and the potential for flash floods and damaging winds. In this case, it will feed high precipitation rates in the form of freezing rain, sleet and snow, which could be accompanied by thunder and lightning.

This storm, coming in the wake of previous recent wild ones in various parts of the country, will slice east into the Northeast on Friday and Friday night. Then following in this one's footsteps there'll be another energetic system developing in the Midwest and Northeast on Saturday night and Sunday, although the details of that one will be different.

UPDATE 11:30 PM CDT THU DEC 18

The storm is cranking up ... here's a sampling of a few recent reports ...

859 PM 12/18/2008 TRENTON MO
ICE STORM

0.25 INCHES OF ICE ACCRETION...MOST OF WHICH OCCURRED WITH THUNDERSTORMS JUST BEFORE 9 PM.

1042 PM 12/18/2008 MACOMB IL
FREEZING RAIN

ESTIMATED .25 TO .5O INCHES ON BRANCHES AND EXPOSED SURFACES WITH BRANCHES BENDING AND SOME BREAKING LIMBS. HEAVY FREEZING RAIN CONTINUES.


1047 PM 12/18/2008 S MOUNT AUBURN IA
HEAVY SNOW

HEAVY THUNDERSNOW. 1.5 INCHES MAINLY IN THE LAST HOUR.

MONDAY DECEMBER 22 UPDATE

In addition to the severe icing in some locations, esp. central Illinois, the storm went on to produce a wide east-west band of significant snow accumulations:

#2 of the latest 1-2 punch produced heavy snow and increasing wind in New England on Sunday ...

... while this observation from Saturday evening exemplifies the potent storm that Portland, Oregon had:

KPDX 210153Z 11023G30KT 1/2SM SN M06

Translation: biting ESE winds sustained at 23 knots (26.5 mph) with gusts to (34.5 mph), snow coming down hard and reducing visibility to 1/2 mile, and temperatures well below freezing (minus 6 C); what's more, the snow later changed to freezing rain.