Significant Weather Events

Flooding Risks from Tropical Moisture Across the Southeast and Southwest

Significant Weather Events

Historic snowfall event in the Gulf Coast region

Historic snowfall event in the Gulf Coast region

December 4, 2009 Snow

click on map for larger image

A powerful and deep upper level trough of low pressure moved quickly across Texas and Louisiana on Friday, December 4, 2009. At the same time, a strong area of surface low pressure developed in the western Gulf of America. Meanwhile, a cold Arctic airmass filtered southward through the southern United States. All of these factors combined to produce a large area of rain along the Texas coast during the morning of December 4, 2009, which then spread northeastward across Southeast Texas into Central and Southern Louisiana during the late morning and afternoon hours. As temperatures gradually cooled during the afternoon and evening, the rain began to mix with and eventually change over to snow from west to east across the area. Snow fell only briefly near the immediate Gulf Coast, but snow was seen for many hours across inland areas of Southeast Texas and Southwest Louisiana.

As the winter weather event ended on Friday evening, December 4, totals snow accumulations ranged from just a dusting up to 3 inches. Since any snowfall is rare across Southeast Texas and Southwest Louisiana, this event set several snowfall records:

Alexandria

Second earliest measurable snowfall on record for the fall/winter season (record: 12/02/1896)
Fifth highest snowfall total (0.2") for the month of December
Only fifth time in history measurable snow has occurred in December

Lake Charles

Earliest measurable snowfall on record for the fall/winter season (old record: 12/11/08)
Second highest snowfall total (0.2") for the month of December
(old records: 0.4" on 12/11/08, 0.2" on 12/22/89)
Only third time in history measurable snow has occurred in December

Lafayette

Earliest measurable snowfall on record for the fall/winter season (old record: 12/11/08)
Second highest snowfall total (0.3") for the month of December (old record: 1.0" on 12/11/08)
Only second time in history measurable snow has occurred in December

12/4/09 Snow Reports
TEXAS		LOUISIANA
5 N Silsbee	2.5"	Eunice	3.0"
Kountze	2.0"	Fort Polk	2.5"
Wildwood	1.2"	Rosepine	2.5"
1 NNW Lumberton	1.0"	Anacoco	2.0"
Beaumont (Water Plant)	0.5"	5 ENE Dequincy	2.0"
Beaumont (KFDM-TV)	0.3"	Elmer	2.0"
7 S Woodville	0.1"	Leesville	2.0"
SE TX Regional Airport	T	Ville Platte	2.0"
Jasper	T	Lacassine	1.5"
		Welsh	1.5"
		Alexandria 911 Center	1.0"
		Bell City	1.0"
		Carencro	1.0"
		Plaisance	1.0"
		Jennings	0.8"
		Lake Arthur	0.5"
		5 SSE Dequincy	0.3"
		7 SW Boyce	0.3"
		Lafayette	0.3"
		Alexandria Int'l Airport	0.2"
		Lake Charles	0.2"
		Moss Bluff	0.2"
		New Iberia	0

National Ice Center Snow Cover Maps
*December 4, 2009*				*December 5, 2009*

Storm Prediction Center Mesoscale Discussions

MCD2196

MESOSCALE DISCUSSION 2196
NWS STORM PREDICTION CENTER NORMAN OK
1020 AM CST FRI DEC 04 2009

AREAS AFFECTED...UPR TX CST...SWRN-CNTRL LA AND EXTREME SWRN MS

CONCERNING...HEAVY SNOW

VALID 041620Z - 042215Z

12Z OBSERVED SOUNDING AT CRP AND RECENT AIRCRAFT OBS SUGGEST THAT MODELS WERE TOO WARM WITH THE THERMODYNAMIC PROFILE ACROSS THE REGION. IN ADDITION...DPROG/DT OF THE SHORT-TERM MODEL SUITE INDICATE A FASTER PROGRESSION OF THE APCHG UPR TROUGH OVER THE TX BIG BEND REGION. THE RESULT WILL BE AN EARLIER ONSET OF PCPN ACROSS THE DISCUSSION AREA AND A MORE RAPID PHASE CHANGE TO SNOW...AN UNUSUAL EVENT FOR EARLY DECEMBER AT THIS LATITUDE.

WEBCAMS IN THE SERN METRO HOUSTON AREA INDICATE THAT RAIN HAS CHANGED TO SNOW AND PHASE CHANGE TO SLEET SW OF KCRP LEND CREDENCE TO THE ABOVE THINKING OF A COLDER-THAN-FCST TROPOSPHERE. AS THE UPR TROUGH CONTINUES ACROSS THE REGION...PWAT VALUES OF 0.8-1" WILL BE DRAWN NEWD ATOP THE SFC COLD DOME...BOOSTING PCPN RATES/EFFICIENCY. AS PCPN RATES INCREASE...WET-BULB PROCESSES WILL MAKE FOR DEEP AOB 0 DEG C ISOTHERMAL PROFILES AND RESULT IN A CHANGEOVER TO ALL SNOW.

AT THIS POINT...HEAVIEST SNOWFALL THROUGH EARLY EVENING WILL LIKELY OCCUR IN A 40-60 MILE WIDE BAND EXTENDING FROM THE E AND SERN SUBURBS OF HOUSTON NEWD TO 40 N OF KLCH TO ABOUT 25 S OF KHEZ. HERE...STRONGEST 850-700 MB FRONTOGENETIC FORCING WILL BECOME JUXTAPOSED WITH INSTABILITY ALOFT AND RESULT IN THE NARROWING OF THE HEAVIEST PCPN BANDS. SNOWFALL RATES MAY REACH 1/2-1" PER HOUR AT TIMES.

..RACY.. 12/04/2009

MCD2197

MESOSCALE DISCUSSION 2197
NWS STORM PREDICTION CENTER NORMAN OK
1213 PM CST FRI DEC 04 2009

AREAS AFFECTED...S/CNTRL LA/MS

CONCERNING...HEAVY SNOW

VALID 041813Z - 042215Z

CLIMATOLOGICALLY RARE SNOW EVENT WILL CONTINUE TO DEVELOP EWD THROUGH THE EVENING...WITH RAIN TRANSITIONING TO SNOW OVER PARTS SRN/CNTRL LA AND MS. RATES NEAR .5 TO 1 INCH AN HOUR ARE POSSIBLE.

A BROAD UPPER TROUGH ACROSS THE CNTRL CONUS WILL CONTINUE TO PROGRESS EWD...WITH NLY/NWLY FLOW ALOFT DRIVING A CONTINENTAL POLAR AIRMASS SEWD TO THE GULF COAST AS THIS OCCURS. CONFLUENT FLOW ALOFT AROUND THE BACKSIDE OF THE UPPER TROUGH IS LEADING TO SURFACE PRESSURE RISES ACROSS MUCH OF TX/LA. THIS WILL ALLOW A SURFACE HIGH CENTERED ROUGHLY NEAR N CNTRL TX TO DEVELOP SEWD...WITH STRONG COLD AIR ADVECTION LEADING TO FALLING SURFACE TEMPERATURES THROUGH THE AFTERNOON/EVENING. 18Z LCH SOUNDING INDICATES A SHALLOW LAYER OF SUBSATURATED AIR BELOW 900 MB WHERE WET-BULB PROCESSES WILL LIKELY CONTRIBUTE TO FALLING SURFACE TEMPERATURES. WET-BULB EFFECTS/COLD AIR ADVECTION HAVE BEEN CONTRIBUTING TO A FASTER TRANSITION FROM RAIN TO SNOW THAN MODEL FORECASTS INDICATED ACROSS SERN TX/SWRN LA. MOISTURE TRANSPORT NEAR THE COAST WITH PWAT VALUES NEAR .75 TO 1 INCH WILL CONTRIBUTE TO INCREASED PRECIPITATION RATES/EFFICIENCY. ALTHOUGH GROUND TEMPERATURES ARE ABOVE FREEZING...STEADY SNOWFALL WILL LIKELY LEAD TO ACCUMULATION.

..HURLBUT.. 12/04/2009

MCD2198

MESOSCALE DISCUSSION 2198
NWS STORM PREDICTION CENTER NORMAN OK
0522 PM CST FRI DEC 04 2009

AREAS AFFECTED...SERN TX...MUCH OF LA...AND CENTRAL MS

CONCERNING...HEAVY SNOW

VALID 042322Z - 050515Z

CLIMATOLOGICALLY RARE SNOWFALL EVENT IS EXPECTED TO TRANSLATE E-NEWD ACROSS LA AND INTO MS THROUGH 06Z...WHILE COMING TO AN END DURING THE 00-03Z TIME FRAME OVER SERN TX.

UPPER LEVEL TROUGH EVIDENT IN WV IMAGERY IS FORECAST TO MOVE EWD FROM TX INTO MS/AL BY 12Z. AS THIS OCCURS...ZONE OF LARGE SCALE ASCENT AHEAD OF THE TROUGH COMBINED WITH SLOPED REGION OF 850-500 MB FRONTOGENESIS WILL LIKELY MAINTAIN A BROAD SHIELD OF PRECIPITATION OVER LA AND MS THROUGH 06Z. RECENT SURFACE OBSERVATIONS INDICATE THAT RAPID WET BULB COOLING TO AROUND 32 DEG F IS OCCURRING IN CONJUNCTION WITH STRONGEST UVV/S AND PRECIPITATION RATES...RESULTING IN A CHANGE FROM RAIN TO SNOW...AND MODEL GUIDANCE INDICATES THIS TREND WILL CONTINUE AS STRONGEST ASCENT SHIFTS FROM SERN TX ACROSS LA DURING THE 00-03Z TIME PERIOD...AND THEN INTO MS BY 03-06Z. AT THIS TIME...MODEL FORECAST GUIDANCE SUGGESTS SNOWFALL RATES WILL REMAIN UNDER 1 INCH/HOUR...BUT MOIST ISOTHERMAL FORECAST SOUNDINGS THROUGH THE LOWEST 2 KM/S INDICATE WET HEAVY SNOW FLAKES WILL BE POSSIBLE WITH REDUCTION IN VISIBILITIES TO AROUND 1 MILE.

..GARNER.. 12/04/2009

Courtesy of CIMMS Satellite Blog

December 4-5, 2009 Satellite Imagery

Visible Satellite Imagery
compiled by Donovan Landreneau, Forecaster
(click on image for satellite loop)

CIMMS Satellite Blog Imagery
(click on image for satellite loop)

Satellite Analysis Branch/NESDIS Analyses (1431Z & 1655Z)

SATELLITE PRECIPITATION ESTIMATES..DATE/TIME 12/04/09 1431Z
SATELLITE ANALYSIS BRANCH/NESDIS
LATEST DATA USED: GOES-12 1400Z KUSSELSON

LOCATION...LOUISIANA...TEXAS...

ATTN WFOS...LCH...SHV...HGX...FWD...OUN...CRP...EWX...SJT...MAF...
ATTN RFCS...LMRFC...ABRFC...WGRFC...

EVENT...SHORT WAVE SNOW CENTRAL TX...SHORT WAVE RAIN SE TX...

SATELLITE ANALYSIS AND TRENDS...TWO ACTIVE SHORT WAVES CAUSING DIFFERENT TYPES OF PRECIP ACROSS TEXAS THIS MORNING. SHORT WAVE THAT YESTERDAY WAS ACROSS 4 CORNERS (UT-CO-NM-AZ) WAS NOW PUSHING ACROSS W CENTRAL TX AND HAS TEAMED UP WITH ANOTHER SHORT WAVE NOW OVER THE N TX PANHANDLE THAT DROPPED DOWN OVER THE EASTERN ROCKIES THE PAST 18HRS. BEST LIFT JUST AHEAD OF PART OF THE SHORT WAVE TROUGH IN W CENTRAL TX AND RESULTANT BEST PRECIP JUST AHEAD IN BEST MOISTURE IN THAT AREA. TO THE NORTH AND NE...DRIER AIR HAS TO BE MOISTENED BEFORE PRECIP BEGINS. GOOD GENERAL RULE OF THUMB IN THE PAST FOR RAIN/SNOW LINE CAN BE THE 0.40" PW VALUE WITH THE LOWER PWAT VALUES BELOW 0.40" MORE LIKELY TO SNOW (THAN RAIN) AND THE HIGHER PWAT VALUES ABOVE 0.40" MORE LIKELY TO RAIN (THAN SNOW). CURRENT 0.40"LINE ACROSS TX FROM TERRELL/W VAL VERDE TO LAMPASAS IN C TEXAS TO MARION COUNTY IN NE TX. THIS LINE EXPECTED TO SHIFT EAST MORE QUICKLY ACROSS S CENTRAL TX THIS MORNING...CENTRAL TX MIDDAY/AFTERNOON AND STAY ABOUT STATUS QUO ACROSS
NE TX THRU MIDDAY.

OTHER SHORT WAVE TO THE SOUTH AND SE NOW ACROSS MID TX COAST AND LIFTING BEST PRECIP NORTH TOWARD SW LA/UPPER TX COAST. NO CONCERNS FOR SNOW HERE FOR MOST OF THE DAY AS PWAT VALUES WAY ABOVE 0.40" CRITICAL VALUES. THE SHORT WAVE PRODUCING BEST PRECIP HERE CAME OUT OF A SUBTROPICAL SHORT WAVE THAT CROSSED N MEXICO THE PAST 24HRS AND GENERALLY HAS BEEN NORTH OF THE SUBTROPICAL JET THAT NOW RUNS ACROSS CENTRAL TO NE OLD MEXICO AND ACROSS THE CENTRAL Gulf of America. WILL CONTINUE TO MONITOR...

SATELLITE PRECIPITATION ESTIMATES..DATE/TIME 12/04/09 1655Z
SATELLITE ANALYSIS BRANCH/NESDIS
LATEST DATA USED: GOES-12 1645Z KUSSELSON

LOCATION...LOUISIANA...TEXAS...

ATTN WFOS...LCH...SHV...HGX...FWD...EWX...SJT...
ATTN RFCS...LMRFC...WGRFC...

EVENT...CONVECTIVE TAIL DEVELOPING ACROSS INTERIOR SE TX...
BAROCLINIC LEAF SNOWS WITH VORT ACROSS CENTRAL TX...

SATELLITE ANALYSIS AND TRENDS...DYNAMIC COOLING A BIT STRONGER WITH SUBTROPICAL VORT THAT HAS LIFTED NORTHEAST ACROSS SE TX AND NOW DEVELOPING A CONVECTIVE LOOKING BAND SW TO NE AND JUST INLAND FROM THE SE TX COAST. ENOUGH DYNAMIC COOLING HAS LOWERED SFC TO 850MB TEMP AND RAIN THAT HAS CHANGED TO SNOW HAS INCRD TO MODERATE/LOCALLY HVY ALONG
CONVECTIVE BAND. MOST CONCERNED NEXT 1-2HRS ACROSS E BEE/E GOLIAD COUNTY TO WHARTON COUNTY THRU MID AFTERNOON TO THE CENTRAL TO N HARRIS COUNTY THRU EARLY AFTERNOON AND INTO MONTGOMERY COUNTY FOR THE AFTERNOON HRS. IT IS CONCEIVABLE THAT WITHIN THE CONVECTIVE BAND WITHIN ENOUGH TRAINING CELLS THAT MAX SNOW RATES COULD REACH 0.5"-1.0"/HR ON GRASSY SURFACES THRU AT LEAST MID AFTERNOON. TRICKY AS THIS WHOLE AREA SHIFTS EAST THRU LA AS DYNAMIC COOLING WILL DICTATE THE CHANGEOVER AND AMOUNT OF SNOW...ESPECIALLY ACROSS CENTRAL AND N LA FOR THE AFTERNOON INTO EVENING HRS.

OUT WEST IN THE CENTRAL TX AREA AHEAD OF SHORT WAVE MOVING ACROSS THE SE TX PANHANDLE AND W CENTRAL TX...MOSTLY LIGHT SNOW WILL SPREAD EAST AND NE THRU NE TX AND CONCERN FOR POINTS FROM NE TX EASTWARD WILL BE HOW MUCH CONTRIBUTION DEEP MOISTURE COMING NORTH/NE FROM TX COAST WILL HAVE IN STARTING TO PRODUCE ANY MODERATE SNOWS BEFORE THE
PASSAGE OF THE WEST CENTRAL TX SHORT WAVE. WILL CONTINUE TO MONITOR...