Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 271519
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
1019 AM CDT WED APR 27 2016

.UPDATE...
ISSUED AT 1014 AM CDT WED APR 27 2016

HAVE FINED TUNED POPS FOR TODAY AND TONIGHT TO BETTER REFLECT
RECENT CAM RUNS AND DENOTE THE MOST PROBABLE BREAKS IN ANY
PRECIP. FORECAST FOR AFTERNOON CONVECTIVE INITIATION ACROSS
SOUTHWEST IOWA REMAINS ON TRACK WITH RECENT RUNS OF THE HRRR AND
OTHER HIGH RES MODELS BRINGING THE WARM FRONT UP TO HWY 34 BY
21-22Z AND SURFACE OBS MATCHING THIS IDEA WITH THE FRONT STARTING
TO NOSE INTO THE FAR SOUTHWEST TIP OF IOWA. THE BIG QUESTION WILL
BE THE AMOUNT OF CLEARING AND DESTABILIZATION ALONG THE BOUNDARY
EARLY THIS AFTERNOON AS THE DRY SLOT WRAPS NORTHEASTWARD. VISIBLE
SATELLITE DATA DOES SHOW A POCKET OF CLEARING IN SE NEBRASKA
SPREADING EASTWARD AND TRENDS WITH THIS CLEARING WILL BE MONITORED
OVER THE NEXT 3-4 HOURS.

&&

.SHORT TERM.../TODAY/
ISSUED AT 352 AM CDT WED APR 27 2016

CONVECTION TODAY...AND MORE SPECIFICALLY SEVERE POTENTIAL...WILL
LARGELY DEPEND ON WHERE THE WARM FRONT GETS TO. STRONG UPPER LOW
WILL PIVOT FROM WEST CENTRAL KANSAS INTO SOUTHEAST NEBRASKA BY 06Z
THURSDAY. THE SURFACE LOW WILL SLOWLY SHIFT FROM NORTH CENTRAL/
NORTHEAST KANSAS INTO FAR SOUTHWEST IOWA DURING THIS SAME TIME
FRAME. THERE REMAIN MODEL DIFFERENCES ON THE PLACEMENT OF THE WARM
FRONT BUT ALL MODELS PLACE IT INTO AT LEAST AS FAR AS FAR NORTHERN
MISSOURI. THE GFS LIFTS THE FRONT FURTHER NORTH ACROSS SOUTHWEST
IOWA AND AT LEAST ACROSS THE SOUTHERN MOST TWO TIERS OF COUNTIES
IN IOWA WHILE THE NAM IS AT LEAST SIMILAR...IT DOES NOT BRING IT
QUITE AS FAR NORTH ACROSS THE SOUTHWESTERN PORTION OF THE FORECAST
AREA AND IT IS A BIT LATER IN LIFTING THE FRONT INTO FAR SOUTHERN
IOWA.

EITHER WAY...GIVEN THE FRONTAL BOUNDARY AND SUFFICIENT SHEAR...THERE
APPEARS TO BE A WINDOW FROM LATE MID TO LATE AFTERNOON INTO EARLY
EVENING WHERE A FEW SEVERE STORMS ARE POSSIBLE AND A TORNADO WOULD
BE POSSIBLE NEAR THE BOUNDARY. OF COURSE IT REMAINS TO BE SEEN
HOW MUCH WE WILL ACTUALLY DESTABILIZE WITH SHOWERS AND ISOLATED TO
SCATTERED THUNDERSTORMS PERSISTING ACROSS THE SOUTH/SOUTHWEST
NEARLY ALL DAY. THE NAM DOES SUGGEST THIS FIRST BAND OF
SHOWERS/STORMS WILL LIFT NORTH AND EAST ROTATING AROUND THE LOW
WITH A BREAK IN THE AFTERNOON BEFORE STORMS REFIRE IN THE 21Z TO
00Z TIMEFRAME THEN THAT ROTATES NORTHEAST ACROSS THE FORECAST AREA
THROUGH THE EVENING. IF THAT SCENARIO PLAYS OUT WE WOULD CLEAR OUT
SOME AND HAVE SOME TIME TO DESTABILIZE FURTHER ACROSS THE FAR
SOUTH. THE NAM GIVES US A BETTER CHANCE FOR A SEVERE WINDOW.

.LONG TERM.../TONIGHT THROUGH TUESDAY/
ISSUED AT 352 AM CDT WED APR 27 2016

OVERALL THE SITUATION REMAINS VERY SIMILAR TO WHAT WAS EXPECTED
YESTERDAY. A WEAK FLOW AND BLOCKING PATTERN THROUGH THE ROCKIES
WILL KEEP IA IN A VERY SIMILAR WEATHER REGIME THROUGH THE
PERIOD...ESSENTIALLY SEASONALLY COOL AND DAMP. AT ONSET THE
PRIMARY CONCERN WILL BE SEVERE WEATHER EARLY IN THE EVENING...AND
THAT IS VERY CONDITIONAL DEPENDING ON WARM FRONTAL PLACEMENT AND
THE AMOUNT OF INSOLATION. MODEL SOLUTIONS VARY FROM THE FRONT
REMAINING INTO MO /HI RES ARW AND NMMB/ TO WELL INTO SRN IA /GFS/.
00Z ECMWF IS FAIRLY CLOSE TO THE GFS WHILE THE NAM AND NESTED NAM
HAVE MORE OF A NW-SE ORIENTATION...JUST PLACING SWRN SECTIONS IN
THE WARM SECTOR. ALTHOUGH THE FRONT IS CURRENTLY WELL INTO
MO...RECENT 3HR PRESSURE FALLS ARE EXPANDING RAPIDLY AHEAD OF THE
KS SURFACE LOW SO EXPECT THE FRONT TO REACH AT LEAST PART OF THE
FORECAST AREA BY 00Z WITH THE NAM POSSIBLY LOOKING THE MOST
REASONABLE. IT WOULD SEEM THE ONLY SEVERE WEATHER THREAT WOULD BE
A TORNADO OR NOTHING. FORECAST SOUNDINGS DEPICT FAIRLY TALL/SKINNY
CAPES WITH WEAK DEEP SHEAR. THE PRIMARY CONCERN WOULD BE
THERMODYNAMICALLY DOMINATE TORNADOES DRIVEN BY 0-3KM CAPES 100-200
J/KG AT A HIGH PERCENTAGE OF TOTAL MLCAPE...VERY LOW LCLS...AND
ADEQUATE SURFACE VORTICITY NEAR THE BOUNDARY. THUS CANNOT ARGUE
WITH THE CURRENT SPC CATEGORICAL SLIGHT SEVERE OUTLOOK...AND
WOULD NOT OPPOSE THE TOR PROBABILITIES NUDGED A BIT FARTHER NORTH
INTO IA.

ANY SEVERE WEATHER POTENTIAL SHOULD QUICKLY WANE THROUGH THE EVENING
WITH LOSS OF HEATING AS THE UPPER LOW DRIFTS INTO IA. LOBE OF
FORCING NORTH AND EAST OF ITS TRACK WILL KEEP RAIN AND POSSIBLY A
FEW RUMBLES OF THUNDER GOING INTO THE NIGHT WITH HIGHEST POPS NORTH.
THE UPPER LOW AND ASSOCIATED FORCING WILL EXIT THU WITH POPS
DIMINISHING WEST TO EAST LEADING TO MAINLY DRY CONDITIONS THU NIGHT
AND FRI. TEMPS WILL RECOVER LITTLE THU WITH PLENTY OF LOW CLOUDS
AND WINDS BECOMING NLY. OUTSIDE OF POSSIBLE TOWARD THE MO
BORDER...THE DIURNAL SWING WILL MAINLY BE LESS THAN 10F.

THE NEXT SYSTEM FOR THE WEEKEND IS CURRENTLY APPROACHING THE CA/OR
COAST AND THE ECMWF AND GFS ARE FAIRLY SIMILAR WITH ITS PROGRESSION.
BOTH KEEP IA IN THE COOL SECTOR WITH THE SURFACE TRACK TO THE SOUTH.
A LOBE OF PHASED MECHANICAL DPVA/DIVERGENCE ALOFT AND WARM/THETA-E
ADVECTION BELOW SHOULD HAVE NO PROBLEM SPREADING SHOWERS AND
POSSIBLY SOME BELOW SEVERE THUNDER INTO IA BY SAT AND LINGERING
INTO AT LEAST EARLY SUN. THUS HAVE LIKELY POPS GOING AND ONLY
SLOWLY DIMINISHING INTO SUN NIGHT. CENTRAL AND WESTERN IA WILL
AGAIN SEE THE BRUNT OF THIS PRECIP WITH AMOUNTS OF AN INCH OR
MORE. THE FLOW IS WEAK AND NON-DESCRIPT INTO MON AND TUE AND
ESSENTIALLY DRY. WOULD NOT BE SURPRISED TO SEE SOME PERIODS OF FOG
AND/OR STRATUS WITH VERY WEAK SURFACE FLOW AND MOIST GROUND/LOW
LEVELS AFTER THESE TWO SYSTEMS.

&&

.AVIATION.../FOR THE 12Z TAFS THROUGH 12Z WEDNESDAY MORNING/
ISSUED AT 644 AM CDT WED APR 27 2016

MAINLY IFR CIG WITH OCNL LIFR CIGS.  SCT SHRA/TSRA THROUGH 00Z BUT
TIMING OF THE SCT PRECIP DIFFICULT.  I HAVE LEFT VCSH IN FOR MUCH OF
THE TAF PERIOD AND TRIED TO TIME THE BEST CHANCES FOR THUNDER. KDSM
AND KOTM WILL BE MORE DIFFICULT TO FORECAST AS A WARM FRONT WILL
LIFT TO NRN MO AND POSSIBLY INTO SRN IA AND WILL LIKELY BUCKLE A BIT
FURTHER NORTH WEST OF THE I-35 CORRIDOR AS THE SURFACE LOW LIFTS OUT
OF SE NEB.  STRONG STORMS POSSIBLE FM 21-01Z INVOF THE FRONT WITH
MULTIPLE AVIATION HAZARDS POSSIBLE.


&&

.HYDROLOGY...
ISSUED AT 352 AM CDT WED APR 27 2016

RIVER FORECASTS BASED ON 24 HOURS OF QPF STARTING AT 00Z SUGGEST
SIGNIFICANT WITHIN BANK RISES AT SEVERAL LOCATIONS THROUGHOUT
CENTRAL AND WESTERN IOWA...AND THERE IS STILL MODERATE TO HIGH
CONFIDENCE THAT FLOOD STAGES WILL NOT BE REACHED WITH THIS CURRENT
SYSTEM AS RAINFALL AMOUNTS MAY HAVE EVEN BEEN A TOUCH OVER
FORECAST. RFC CONTINGENCY FORECASTS THROUGH 72 HOURS SUGGEST
SIMILAR OUTCOMES WITH THE WEEKEND SYSTEM AS WELL WITH SIGNIFICANT
WITHIN BANK RISES AND JUST AN OUTSIDE POTENTIAL OF REACHING FLOOD
STAGE AT A FEW SPOTS IN THE RACCOON AND DES MOINES BASINS.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

UPDATE...SKOW
SHORT TERM...FAB
LONG TERM...SMALL
AVIATION...FAB
HYDROLOGY...SMALL



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