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12/20-12/21 Moderate/Intense Clipper Event?


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The NAM has nearly 3x the qpf that the RGEM does here. Kind of unsettling.

This whole pattern has been unsettling to me.. :P

But yeah i hear you. I just know to expect the worst. Atleast till that crap in the Atlantic is gone. Thus if i get what the RGEM shows i'll call it a win and be happy.

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Hard to believe this is what the GFS showed just 2 days ago. What a sad change for Iowa and western IL...

BTW, we need more posters from Iowa. Seems to be a void in this region relative to other regions...

12zgfsensemblep12120.gif

Yeah models struggled with this event. Still struggling. It is a tough setup for them to get right. We are lucky they can even do as well as they do. But yeah, a huge change no doubt.

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12Z NAM significantly wetter. Still didn't grab the snow hitting the ground over Ern Iowa or IL that well, but did much better than the previous run with initialization. Combined with GFS, I may be a bit low on some of my previous calls, especially for RFD.(DVN looks just fine still.)

Nevermind, appears the NAM is doing its phase-shift shenanigans again, though I would imagine the errors it causes are fairly small 12 hours out.

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LOT

WATER VAPOR IMAGERY SHOWS WELL DEFINED SHORTWAVE AND ASSOCIATED JET

STREAK NOSING EASTWARD FROM THE ROCKIES INTO THE NORTHERN PLAINS.

REGIONAL PROFILERS AND WSR-88D VWPS SHOW FLOW BACKING IN RESPONSE

TO THE HEIGHT FALLS TAKING PLACE AHEAD OF THIS SHORTWAVE. THE

BACKING LOW AND MID LEVEL FLOW IS RESULTING IN INCREASINGLY STRONG

ISENTROPIC ASCENT AND A BLOSSOMING AREA OF LIGHT TO MODERATE SNOW

OVER THE PLAINS.

AS THIS SHORT WAVE TRANSLATES EASTWARD ALONG THE CANADIAN BORDER

TODAY LOOK FOR THE STRENGTHENING WARM AIR ADVECTION TO RESULT IN A

SHARPENING UP OF LOW/MID LEVEL FRONTAL BOUNDARY. GFS AND ECMWF IN

PARTICULAR DEVELOP AN IMPRESSIVE FRONTOGENETIC CIRCULATION THIS

AFTERNOON FROM MINNESOTA SOUTHEAST INTO NORTHERN ILLINOIS. VERTICAL

CROSS SECTIONS THROUGH THIS FRONTAL CIRCULATION DO INDICATE A

REGIONAL OF WEAKLY NEGATIVE SATURATED EPV (AN INDICATION OF WEAKER

STABILITY/SLIGHT INSTABILITY) WITHIN THE REGION OF MAXIMUM

OMEGA...SUGGESTING THAT A RATHER NARROW BUT IMPRESSIVE BAND OF

FRONTOGENTICALLY DRIVEN SNOW COULD DEVELOP BY MID AFTERNOON OVER OUR

WESTERN CWA. THIS LOW/MID LEVEL FRONTAL CIRCULATION IS EXPECTED TO

TRANSLATE EASTWARD ACROSS THE CWA LATE THIS AFTERNOON INTO THE EARLY

EVENING...LIKELY RESULTING IN A PERIOD OF RATHER HEAVY SNOWFALL.

CONTINUED BROAD/DEEP ISENTROPIC ASCENT SHOULD MAINTAIN A BROAD SWATH

OF LIGHT SNOW ACROSS THE REGION THROUGH AT LEAST THE FIRST HALF OF

TONIGHT...WITH SNOW TAPERING OFF FROM WEST TO EAST AFTER MIDNIGHT AS

DRIER AIR ALOFT LIMITS CLOUD DEPTH AND LIKELY LEAVES STRATUS DECK

VOID OF THE NECESSARY ICE NUCLEI TO PRODUCE SNOW. SET UP CERTAINLY

LOOKS FAVORABLE FOR AREAS OF FREEZING DRIZZLE TO DEVELOP BEHIND THE

SNOW WITH FORECAST SOUNDINGS SHOWING SATURATED LAYER BELOW 850MB TO

NEAR THE SURFACE INDICATIVE OF STRATUS DECK COMPOSED OF SUPER COOLED

DROPLETS WITH SUFFICIENT WIND SHEAR WITHIN THE CLOUD LAYER TO

PROMOTE DROPLET GROWTH THROUGH COLLISION/COALESCENCE. IN

ADDITION...LOW LEVEL WARM/MOIST ADVECTION OVER TOP THE SNOW PACK

SHOULD ALLOW THE STRATUS DECK TO BUILD TO NEAR THE SURFACE WITH

DRIZZLE/FOG A GOOD BET TUESDAY MORNING. CONTINUED POSITIVE THETA E

ADVECTION SHOULD EVENTUALLY PUSH TEMPS ABOVE 32F TUESDAY WITH

FREEZING DRIZZLE TRANSITIONING TO JUST PLAIN DRIZZLE BEFORE ENDING

IN THE AFTERNOON WITH THE PASSAGE OF THE TROUGH.

NOT REALLY THINKING SNOW AMOUNTS WILL REACH ADVISORY CRITERIA OVER

MOST OF THE CWA...WITH GENERALLY 2-4" NORTHERN PORTIONS OF THE CWA

TAPERING TO LESS THAN 2 INCHES SOUTH OF THE ILLINOIS AND KANKAKEE

RIVERS. DESPITE AMOUNTS BELOW ADVISORY CRITERIA NOT PLANNING TO DROP

THE ADVISORY AT THIS TIME FOR A COUPLE OF REASONS. FIRST...THERE

DOES APPEAR TO BE A PRETTY GOOD CHANCE OF AT LEAST A PERIOD OF

FREEZING DRIZZLE LATE TONIGHT INTO TUESDAY MORNING. HOWEVER....WITH

TEMPS NEAR FREEZING AND MOST ROADS ALREADY WET/TREATED THE FZDZ WILL

PROBABLY ONLY ACCUMULATE ON ELEVATED OBJECTS MAKING IT A PRETTY LOW

IMPACT. SECOND AND MORE IMPORTANTLY...THERE COULD BE A BAND OF

FAIRLY IMPRESSIVE SNOW IN THE CWA DURING RUSH CREATING PRETTY

TREACHEROUS CONDITIONS...SO CANCELING THE ADVISORY COULD POTENTIALLY

SEND A MIXED MESSAGE. SINCE ACCUMULATING SNOWFALL IS EXPECTED TO

START UP DURING THE AFTERNOON/EARLY EVENING WILL BE MOVING UP THE

START TIME OF THE ADVISORY TO BETTER MATCH EXPECTED HIGHER IMPACT

WEATHER CONDITIONS.

IZZI

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The good news for ORD is that they shouldn't have any dry air issues to deal with. It's currently 16/11.

The bad news for ORD is that the 12z NAM (fwiw) has the 546 thickness line pass through for the last half of the event. Even if ORD manages to get 0.30" of liquid (an optimistic projection to begin with), at least 0.10" of this could be non-snow. The more I follow these events, it makes you realize that snow is not an easy process; many things need to go right. For example, you'd think we wouldn't have precip-type issues with such an antecedent cold airmass in place and good snowcover in the midwest for mid-Dec...

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