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WidreMann told us it was gonna snow thread


LithiaWx

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Gsp discussion is out. Can't paste from phone. No watches or advisories issued.

 

TONIGHT...THE MAIN FORECAST CHALLENGE WILL CONCERN THE DURATION OF

THE PRECIPITATION ACROSS THE TN BORDER COUNTIES OVERNIGHT. MOST

GUIDANCE INDICATES THAT PRECIP SHOULD END ACROSS THE TN LINE BY

MIDNIGHT. GIVEN THE LIMITED MOISTURE AND WEAK FLOW...IT SEEMS

REASONABLE TO DROP POPS TO CHC TO SCHC...BEFORE INCREASING TOWARD SUNRISE.

HOWEVER...BEFORE THE TN BORDER PRECIP DECREASES...CAA SHOULD RESULT

IN A TRANSITION OVER TO SNOW. LIMITED MOISTURE AND LOW SLR SHOULD

SUPPORT ONLY LIGHT ACCUMULATIONS OVERNIGHT.  MIN TEMPS SHOULD RANGE

FROM SUB FREEZING ACROSS THE MTNS TO MID 30S EAST.

SATURDAY...FORECAST IS BECOMING QUITE INTERESTING WITH LATEST RUNS

OF THE GUIDANCE. NO MATTER WHAT THE GUIDANCE HOLDS...WE WILL HAVE

WARM SOIL TEMPERATURES EAST OF THE MTNS. LATEST SOIL OBS ACROSS SC

PIEDMONT RANGE IN THE MID TO UPPER 40S. HOWEVER...BRIDGES AND

OVERPASSES WILL REMAIN MUCH COOLER THAN THE SOIL. THAT BEING

SAID...THE CONSENSUS OF THE MODELS INDICATE THAT A NEUTRAL TO

NEGATIVE TILTED H5 TROF WILL PIVOT OVER THE REGION ON SAT. GFS QG H5

UPWARD VERTICAL VELOCITIES PEAK OVER 40 MB/HR ACROSS THE FA AT

18Z...WITH VALUES OVER 30 MB/HR ARRIVING FROM NRN GA AFTER 12Z. GFS

FORECAST SOUNDINGS DURING THE MORNING INDICATE THAT NEARLY SATURATED

CONDITIONS WILL DEVELOP FROM 3 FKT TO NEAR 20 KFT...WITH A DEEP

LAYER WITHIN THE IDEAL SNOW GROWTH RANGE. CYCLOGENESIS OFF THE NC

COAST ON SAT...SHOULD RESULT IN A BAND OF FRONTOGENESIS ACROSS THE

PIEDMONT DURING DAYLIGHT HOURS. IN ADDITION...850MB TO 500MB

SATURATED EPV BECOME NEGATIVE ACROSS THE PIEDMONT AND PORTIONS OF

THE FOOTHILLS DURING THE EARLY AFTERNOON. THE COMBINATION OF THE

DEEP MOISTURE...FORCING...COLD PARTIAL THICKNESSES AND SNOW GROWTH

TEMPERATURES SHOULD PLACE THE PIEDMONT UNDER A STRONG SNOW PRODUCING

ENVIRONMENT.

THE LIMITS TO ACCUMULATING SNOWFALL WILL BE THE MILD SOIL

TEMPERATURES AND ABOVE FREEZING AIR TEMPERATURES. ASSUMING THAT WET

BULB WILL BRING AFTERNOON 2M TEMPS TO THE MID 30S...BELOW MOST

ENSEMBLE MEMBERS AND MOS...ACCUMS SHOULD BEGIN ACROSS THE PIEDMONT.

THE MTN AREAS ARE NOT LIMITED BY WARM SOIL OR AIR

TEMPS...THEREFORE...THE MTN AREAS SHOULD ACCUMULATE SNOW AS LONG AS

IT IS FALLING. BASED ON THE LATEST TIMING AND THINKING WITH THIS

FORECAST PACKAGE...SEVERAL INCHES OF SNOW IS POSSIBLE ACROSS THE

MTNS...WITH AROUND AN INCH ALONG THE I-85 CORRIDOR FROM KGMU TO KCLT

METRO AREA.

 

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Nam still perplexes me with what it does with the phased energy from 30-33 it has the energy on the GA SC border and then instead of swinging it northeast like the GFS it has it go southeast from there. Someone mentioned convective feedback problems earlier I am wondering if that is the problem with the Nam still. That and there is like a million vortexes in the atmosphere as Bean alluded to a little earlier in the run.

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From a forecasting standpoint, I don't know how anyone could have any confidence in a particular accumulation prediction. Those shortwaves end up buzzing around like angry bees. Subtle, 50-mile shifts in their interaction could lead to 100-200 miles shifts in a QPF axis and affect totals by .2-.3. What is becoming clear is there will be some sort of area of .25ish QPF from this system. Where it will end up is anybody's guess.

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Boy, that's not going to be needed if the latest NAM is right.  There's nothing west of CLT -> RDU in NC on the precipitation fields.

 

Don't worry about the NAM.  It hasn't had a good QPF forecast since December.  Last Friday it suggested we would get 1.2" of rain that night, while the GFS had us less than .4".  Actual amount was .38".  And we all know how the NAM was way off with the upper level low system back in January (while the GFS essentially nailed it).

 

Until the NAM can prove it can be accurate again within 24 hours, I'm not putting much stock in it.

 

Notice GSP's discussion didn't talk about the NAM at all.  And for good reason.

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