Area in question for snow threat.

NWS/HPC/NOAA

First some thoughts from the NWS:

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE BIRMINGHAM AL
1009 PM CST SUN FEB 28 2010

ALZ011>015-017>038-011400-
BIBB-BLOUNT-CALHOUN-CHAMBERS-CHEROKEE-CHILTON-CLAY-CLEBURNE-COOSA-
ETOWAH-FAYETTE-GREENE-HALE-JEFFERSON-LAMAR-MARION-PERRY-PICKENS-
RANDOLPH-SHELBY-ST CLAIR-SUMTER-TALLADEGA-TALLAPOOSA-TUSCALOOSA-
WALKER-WINSTON-
1009 PM CST SUN FEB 28 2010

...WINTER PRECIPITATION POSSIBLE MONDAY NIGHT INTO TUESDAY...

A STRENGTHENING LOW PRESSURE SYSTEM WILL CROSS THE NORTHERN GULF
OF MEXICO MONDAY AND MOVE INTO NORTHERN FLORIDA BY TUESDAY AFTERNOON.
AHEAD OF THIS SYSTEM...MOISTURE WILL SPREAD INTO CENTRAL ALABAMA
MONDAY AFTERNOON RESULTING IN WIDESPREAD RAINFALL. COLDER AIR WILL
FILTER INTO THE AREA MONDAY NIGHT FROM THE NORTHWEST...RESULTING
IN THE POSSIBILITY OF WINTER WEATHER.

AT THIS TIME...RAIN IS EXPECTED TO MIX WITH OR TRANSITION TO SNOW
STARTING EARLY TUESDAY MORNING...AND THEN CONTINUE THROUGH THE
DAY ON TUESDAY...EVENTUALLY ENDING FROM WEST TO EAST TUESDAY
AFTERNOON. THE BEST CHANCE FOR SNOW IS ALONG AND NORTH OF A LINE FROM
EUTAW...TO CLANTON...TO ROANOKE.

THE CURRENT FORECAST IS FOR ONE INCH OR LESS OF SNOW IN THESE AREAS.
HOWEVER...THIS FORECAST IS BASED ON TEMPERATURES STAYING JUST WARM
ENOUGH SO THAT MOST OF THE SNOW THAT DOES FALL WILL QUICKLY MELT.
HOWEVER...IT WOULD TAKE A TEMPERATURE DROP OF JUST 2 OR 3 DEGREES TO
CAUSE THESE AMOUNTS TO INCREASE SIGNIFICANTLY.

EVEN AT THIS RELATIVELY LATE JUNCTURE...THERE IS STILL CONSIDERABLE
UNCERTAINTY IN THE FORECAST. MINOR CHANGES IN THE STORM TRACK AND
INTENSITY COULD HAVE A CORRESPONDING MAJOR IMPACT ON THE POSSIBILITY
OF WINTER PRECIPITATION. THE RAIN/SNOW TRANSITION LINE AND
ACCUMULATION POTENTIAL WILL LIKELY NEED TO BE REFINED WITH FUTURE
FORECASTS. CENTRAL ALABAMA SHOULD STAY TUNED FOR UPDATES ON THIS
DEVELOPING WINTER WEATHER SITUATION.

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE PEACHTREE CITY GA
353 AM EST MON MAR 1 2010

GAZ001>009-011>016-019>025-027-030>035-041>045-012100-
DADE-WALKER-CATOOSA-WHITFIELD-MURRAY-FANNIN-GILMER-UNION-TOWNS-
CHATTOOGA-GORDON-PICKENS-DAWSON-LUMPKIN-WHITE-FLOYD-BARTOW-
CHEROKEE-FORSYTH-HALL-BANKS-JACKSON-MADISON-POLK-PAULDING-COBB-
NORTH FULTON-GWINNETT-BARROW-HARALSON-CARROLL-DOUGLAS-
SOUTH FULTON-DEKALB-
353 AM EST MON MAR 1 2010

...WINTRY PRECIPITATION EXPECTED ACROSS MOST OF NORTH GEORGIA
TONIGHT THROUGH TUESDAY NIGHT...

A STORM SYSTEM DEVELOPING OVER TEXAS WILL MOVE EAST ACROSS THE
NORTHERN GULF OF MEXICO TODAY. THIS LOW PRESSURE SYSTEM WILL BEGIN
TO SPREAD PRECIPITATION ACROSS THE SOUTHEAST TONIGHT.

AT THIS TIME...IT APPEARS THAT PRECIPITATION WILL BEGIN AS RAIN
ACROSS MOST OF THE AREA...POSSIBLY MIXING WITH SNOW IN THE MOUNTAINS
AT THE ONSET.

TEMPERATURES ON TUESDAY WILL WARM INTO THE UPPER 30S ACROSS MOST
OF NORTH GEORGIA...THOUGH THE MOUNTAINS WILL LIKELY REMAIN IN THE
LOW TO MID 30S. THE MOST PRECIPITATION WILL FALL IN CENTRAL
GEORGIA IN THE FORM OF ALL RAIN. ACROSS NORTH GEORGIA...
PRECIPITATION ON TUESDAY IS EXPECTED TO BE A MIX OF SNOW AND
RAIN...THOUGH BECAUSE OF THE WARM SURFACE TEMPERATURES THE SNOW
SHOULD MELT AS IT REACHES THE GROUND ON TUESDAY. THE HIGHER
ELEVATIONS OF THE MOUNTAINS...HOWEVER...WILL LIKELY SEE ALL SNOW
AND SOME ACCUMULATIONS DURING THE DAY.

TEMPERATURES WILL DROP TO NEAR OR JUST BELOW FREEZING ACROSS MOST
OF NORTH GEORGIA TUESDAY NIGHT...ALLOWING ANY MIXTURE TO CHANGE
OVER TO ALL SNOW WITH POSSIBLY SOME BRIEF LIGHT ACCUMULATIONS...
MAINLY NORTH OF THE INTERSTATES 20 AND 85 CORRIDORS...BEFORE THE
SYSTEM EXITS.

ANY SNOW THAT MANAGES TO STICK WILL GENERALLY AMOUNT TO LESS THAN
A HALF INCH ACROSS MOST OF NORTH GEORGIA...MAINLY NORTH OF A LINE
FROM CARROLLTON TO ATLANTA TO JEFFERSON. IN THE MOUNTAINS...
ESPECIALLY AT HIGHER ELEVATIONS ABOVE 1500 FEET...AN INCH OR TWO
MAY BE POSSIBLE.

THERE IS STILL SOME UNCERTAINTY WITH THIS SYSTEM. SMALL
ADJUSTMENTS IN THE STORM TRACK MAY MAKE SIGNIFICANT CHANGES IN
TEMPERATURES...AND THUS IN THE POTENTIAL SNOWFALL ACCUMULATIONS.
RESIDENTS OF NORTH GEORGIA ARE URGED TO MONITOR FURTHER UPDATES.

Meteorologist Coleman at Huntsville NASA Center:

this is one of the most difficult forecasts we've seen in a while. An
upper-level system will move out of Texas today and help develop a low
pressure area in the northern Gulf. This low will be in the Florida
panhandle by midnight tonight, and likely strengthen as it moves
northeast. And, temperatures at 850 mb (the "critical level", near
4-5,000 feet, that typically is the rain/snow line) are below freezing.
As an old friend said, "a storm to be proud of." But, despite the
almost perfect dynamic setup for snow, there are some obstacles that
could limit, or even prevent, significant accumulation of snow in
Alabama.

All 5 of the models we often look at, the GFS, NAM, Canadian, European, and UKMET show an area of precipitation moving across central and south Alabama tonight and tomorrow. One thing that makes the forecast difficult is precipitation amount. There will be large differences in precipitation totals over short distances from north-to-south with this storm...one spot along I-65 will get 0.5″ of liquid equivalent precipitation, while another spot 50 miles up the road could get almost nothing. The GFS says BHM will get about 6″ of snow, and the NAM says we will get none. The exact track of a low is hard to pin down even this close to the event, and yet with this one it is very important.

The other challenge is the low-level temperature field. It will be cold enough aloft at Birmingham, Tuscaloosa, and Anniston for the precipitation to fall as snow, at least aloft. However, the models indicate that temperatures will climb to near 50 today, and the melting of the falling snow (causes cooling) will not be enough to overcome the warm temperatures we build up today. The heavier the precipitation, the colder.

The bottom line is that confidence in the forecast is still low for an event only 24 hours away. However, rain is already moving across Texas, and high clouds are moving into Alabama this morning. The key thing may be amount of sunshine and warming today. If it stays below 45, snow becomes more likely. It appears to me that it will be difficult for there to be no precipitation in BHM, or even in Cullman, with a system of this strength. So, rain will likely begin tonight, and slowly mix with and change over to snow by Tuesday morning. If the sun comes out for several hours and it gets warm today, it will look pretty falling, but melt quickly and probably not even accumulate, except maybe in higher elevations. If it stays cold today, there could be a band between Birmingham and Clanton where up to 3″ of snow falls.

NWS South Carolina:

AS THIS POTENTIAL WINTER STORM APPROACHES...CONFIDENCE IN HEAVY SNOW
ONLY MARGINALLY INCREASED WITH THE LATEST MODEL SUITE. THE 00Z GFS
IS STILL THE MOST BULLISH ON COLD AIR AND QPF...PAINTING WARNING
CRITERIA SNOW ACROSS THE ENTIRE THE FA. MEANWHILE...THE
NAM/ECMWF/SREF ARE ALL MUCH DRIER AND A BIT WARMER (DUE TO SLIGHTLY
SLOWER STORM EVOLUTION...AND HENCE LESS DYNAMICAL COOLING THAN THE
GFS. THE 00Z GEM HAS COME IN CLOSE TO THE GFS ON QPF/SNOW AMTS (AND
IS EVEN A LITTLE FASTER ON PRECIP ONSET AHD OF THE SYS. ALSO...THE
CIPS ANALOG OF 25 MAR 1983...WHICH PRODUCED A SWATH OF 6-12" OF SNOW
ALONG THE I-85 CORRIDOR ONLY SEEMS TO BE STANDING OUT MORE WITH EACH
GFS RUN. SO WITH THIS ALL TAKEN INTO ACCOUNT...FEEL THAT THERE IS
ABOUT A 50% CHC OF WARNING CRITERIA SNOW IN A SWATH SOMEWHERE IN THE
FA ON TUESDAY. SO WILL ISSUE A WINTER STORM WATCH FOR ALL COUNTIES
SOUTH OF I-40. BASED ON THE CURRENT STORM TRACK...A GFS RA/SN LINE
ALONG SRN FRINGE OF THE FA...AND HPC QPF (WITH 10:1 SLR)...THE
CURRENT FCST IS FOR 2-4" SNOW ACCUM FROM THE SW NC MTNS...ALONG THE
I-85 CORRIDOR OF THE UPSTATE...TO THE CHARLOTTE METRO AREA. THIS IS
BY NO MEANS SET IN STONE. IN FACT...THERE IS STILL A CHC THAT THE
NAM COULD BE CLOSER TO REALITY...AND A WARM SFC LAYER BASICALLY
PRECLUDES ANY SNOW ACCUM AT ALL.

MY THOUGHTS (as heard on the radio the past 48 hours):

So there you have it, a piece of cake! ;)

As a side note history tells us snow can fall with surface temps as high as 41 proof that it's how "deep" the above 32 air is that matters in the end, deep = rain shallow = snow.

We have to watch this one closely and not let our guard down yet. While more rain or melted snow is favored by me now as the safer forecast, it has potential to be a big snow maker, so it should not be dismissed out of hand. Only a couple degrees C and about 40 miles in storm track separate ATL from a cold rain with some melting snow, and a significant wet snow storm Tuesday. So obviously a lot can change at the last minute. This is probably as close a call as last March 1st and an infamous Atlanta storm back in March of 1983. In fact, IF the computers were showing the lowest 1-2,000 feet of atmosphere just 2 degrees or so cooler I'd be forecasting 1-4 inches for much of the metro. So with only 2 degrees or so between a cold wet day and potential trouble this could go down to the wire. How fast precip falls and just small changes in track will mean a world of difference in the end.

Right now the upper and middle atmosphere (remember our top-down approach homework quiz last week) are projected to be cold enough for all snow. But the models show the boundary layer surface to 1,000 feet or so to be just warm enough to melt a lot of it, producing a rain mix or changeover. If the storm gets strong enough to generate heavy precip fall rates and strong energy lifting dynamics overhead, it would drag down the colder air aloft bringing moderate to heavy snow to much of the area. It looks like a very fine line between the mostly rain look we have in the forecast now OR more snow. A line I'll be tracking on the radio all day and tomorrow.

As of now it looks like mostly rain for most of Atlanta with some nuisance snow late tonight and tomorrow. But separating us from very wet or very white is just those two three degrees and thousand feet or so. The plain hard truth is our computer models and data net just dont have fine enough details to confidently resolve that fine a line. So while more rain than snow is the call at this writing, it could just as easily go the other way! And we may not know until the event is taking place having to nowcast the rain/snow line and amounts as we go or give a last second all clear. This is quite unlike our mid-Feb snow when I was very confident there would be snow, the forecast challenge was only the amount of snow. This time moisture is no question, its the vertical temperature profile that's in question.

MODEL OUTPUT ONLY NOT A FORECAST:

The recent average of the best numerical variants is a liquid equivalent of 0.60 so a good soaking if its all rain. IF that was ALL snow using the standard 10:1 conversion ratio it would be 6 inches of snow.

The mean of the last 10 model runs or so, note these include a range from zero snow to an outrageous 11:

Again these are averages of snow IF all snow

ECMWF 3.5

GFS 3.5

JMA 3

REGEM/GGEM 4.65

NOGAPS 4.5

NAM/DEGEX 5.1

The slightest change can alter everything, a million things need to go right for snow just one wrong for no. Usually when you need a system to "create" its own cold air-- it fails to happen, but when it does happen, it makes for lollipop isolated dumpings of wet flakes.

Since anything I can write hear I will say on the radio tune-in to News Talk 750 WSB AM or click listen live or 5-day forecast at wsbradio.com for the latest update.