Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 290506
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
1206 AM CDT SUN MAR 29 2015

.SHORT TERM /TONIGHT/...
ISSUED AT 301 PM CDT SAT MAR 28 2015

AMPLIFIED YET PROGRESSIVE PATTERN WILL RESULT IN AN UPPER LEVEL
RIDGE SHIFTING ACROSS THE REGION TONIGHT...WITH A SHARP SHORTWAVE
FOLLOWING QUICKLY IN ITS FOOTSTEPS. TOWARDS THE SURFACE...HIGH
PRESSURE CENTERED OVER THE SOUTHERN GREAT LAKES REGION WILL SLIDE
SOUTHEASTWARD THROUGH THE NIGHT AND ALLOW SOUTHERLY RETURN FLOW TO
SET IN THIS EVENING. WAA WILL LIKEWISE COMMENCE ALOFT WITH 850MB
TEMPS RISING FROM -4 TO +4 C OVER DES MOINES BETWEEN 00Z AND 12Z
SUNDAY. TIGHTENING PRESSURE GRADIENT OVER THE STATE OUT AHEAD OF
THE AFOREMENTIONED TROUGH WILL AID IN STABILIZING TEMPS OVERNIGHT
OR EVEN RESULT IN A SLOW WARMING TREND AFTER MIDNIGHT.

MID-LEVEL FORCING IN TANDEM WITH A 850-700MB SHORTWAVE WILL BE
THE INSTIGATOR FOR THE DEVELOPMENT OF SHOWERS OVER CENTRAL IOWA
AFTER 08Z TONIGHT. CANNOT RULE OUT AN ISOLATED THUNDERSTORM WITH
THE NAM IN PARTICULAR DENOTING MUCAPE VALUES OF 100-300 J/KG OUT
AHEAD OF THIS SYSTEM. MOST OTHER SOLUTIONS SHOW LITTLE TO NO
ELEVATED INSTABILITY OVER THE REGION TONIGHT...SO LEFT OUT THUNDER
FOR THE TIME BEING. QPF AMOUNTS SHOULD BE LIGHT GIVEN THE
TRANSIENT NATURE OF THE SYSTEM.

.LONG TERM /SUNDAY THROUGH SATURDAY/...
ISSUED AT 301 PM CDT SAT MAR 28 2015

MAIN CONCERNS DURING THE LONG TERM PERIOD OF THE FORECAST ARE
THREE-FOLD...INCLUDING WINDS ON SUNDAY...CONVECTION POTENTIAL ON
WEDNESDAY AND PRECIP POTENTIAL LATE IN THE WEEK.

PROGRESSIVE SYSTEM SWINGING THROUGH THE REGION ON SUNDAY WILL
BRING CHANCE OF LIGHT QPF...LESS THAN 0.1 IN TOTAL ACROSS THE
CWA. NARROW RIBBON OF FORCING WILL QUICKLY TRANSLATE ACROSS THE
CWA...MOSTLY EXITING THE CWA BY EARLY AFTERNOON. WINDS ABOVE THE
SURFACE WILL BE IMPRESSIVE...AND WILL REACH WIND ADVISORY CRITERIA
ACROSS THE NORTHWESTERN 1/3RD ONCE THEY MIX DOWN TO THE SURFACE.
QUESTION IS EXACTLY WHEN DUE TO DEPARTING CLOUD COVER AND
INVERSION. WINDS ABOVE THE SURFACE ACTUALLY MAX OUT JUST BEFORE
SUNRISE...AS HIGH AS 60-70 KT BETWEEN 850 AND 750 MB. INVERSION
WILL PREVENT FULL MOMENTUM TRANSFER TO THE SURFACE HOWEVER.
PRESENT THINKING IS THAT ADVISORY CRITERIA WINDS WILL BE ALLOWED
TO MIX DOWN TO THE SURFACE BEGINNING AROUND 15Z IN THE FAR
NORTHWEST AND THEN PROGRESSING EASTWARD. THUS WILL BEGIN THE WIND
ADVISORY AT THAT TIME AND CONTINUE THROUGH 22Z...AT WHICH POINT
THE PRESSURE GRADIENT RELAXES AND MIXING DECREASES. PACIFIC AIR
MASS BEHIND THE FRONT WILL RESULT IN MILD TEMPERATURES DURING THE
DAY WITH MAXES IN THE 50S TO LOWER 60S.

UPPER LEVEL FLOW REMAINS NW AFTER THE SUNDAY SYSTEM PASSAGE WITH
THE PACIFIC AIR MASS IN PLACE AT THE SURFACE. H5 RIDGE WILL
DOMINATE THE ROCKIES TO OUR WEST. MODELS BRING A CLIPPER-LIKE
SYSTEM FROM THE NORTHERN DAKOTAS INTO THE SOUTHERN GREAT LAKES
MONDAY INTO TUESDAY MORNING...BUT KEEP THE BEST FORCING AND QPF
NORTH AND NORTHEAST OF THE CWA. THUS WILL CONTINUE THE TREND OF
DRY POPS DURING THAT TIME.

H5 SHORTWAVE COMING ON SHORE IN THE PACIFIC NW TUESDAY WILL
FLATTEN THE ROCKIES RIDGE...AND WILL BECOME REFLECTED AT THE
SURFACE AS A LOW PRESSURE SYSTEM BY WEDNESDAY MORNING EXTENDING
FROM THE DAKOTAS INTO COLORADO. INCREASING PRESSURE GRADIENT WILL
HELP INCREASE WAA AND BRING SEASONALLY WARM TEMPERATURES INTO THE
CWA...WITH HIGHS ON WEDNESDAY IN THE 60S AND 70S. AM CONCERNED
THAT WEDNESDAY/S MAXES MAY NOT BE HIGH ENOUGH GIVEN RUN-TO-RUN
TRENDS HOWEVER.

FRONTAL BOUNDARY IS PROGGED TO MOVE THROUGH THE CWA WEDNESDAY
AFTERNOON AND EVENING. MODELS ARE IN FAIRLY GOOD AGREEMENT
REGARDING ITS TIMING...WHICH AT THIS TIME WILL BE NEARLY OPTIMAL
TO MAXIMIZE THUNDER POTENTIAL FOR THE CWA. FRONT IS EXPECTED TO
ENTER THE NORTHWEST CWA A LITTLE BEFORE 00Z THU AND EXIT A LITTLE
AFTER 06Z THU. MODELS HAVE BEEN CONSISTENT IN ADVERTISING ENOUGH
INSTABILITY FOR TSRA...AND SHEAR ATTM APPEARS SUFFICIENT FOR A
RISK OF SEVERE WX. WILL NOT GET FOCUSED ON MODE ATTM HOWEVER...AS
CONSIDERABLE UNCERTAINTY EXISTS REGARDING MODE SINCE IT IS
ESPECIALLY SENSITIVE TO TIMING THIS TIME OF YEAR WITH RESPECT TO
DIURNAL HEATING. RAINFALL LOOKS TO BE SEASONALLY MODERATE TO
HEAVY...AT LEAST LOCALLY...WITH THIS SYSTEM.

ALTHOUGH GUIDANCE DIVERGES AFTER THE MID-WEEK SYSTEM...THE NEW
ECMWF IS COMING MORE IN LINE WITH THE GFS...WHICH HAS BEEN
ADVERTISING AN ACTIVE SOUTHERN STREAM SYSTEM LATE IN THE WEEK.
PREVIOUS RUN OF THE ECMWF BROUGHT THE SFC SYSTEM ACROSS THE CWA
WITH SEASONALLY MODERATE TO HEAVY QPF...WHILE THE GFS KEPT IT
FARTHER SOUTH. DID NOT CHANGE THE GOING FCST MUCH DURING THIS TIME
DUE TO REMAINING UNCERTAINTY. PACIFIC AIRMASS TEMPS ARE EXPECTED
TO REMAIN WARM ENOUGH...HOWEVER...FOR ANY PRECIP TO BE IN THE
LIQUID PHASE.

&&

.AVIATION...29/06Z
ISSUED AT 1203 AM CDT SUN MAR 29 2015

VFR/CLEAR SKIES ARE ANTICIPATED INTO THE EARLY MORNING HOURS...HOWEVER
ATTENTION WILL TURN UPSTREAM TO PROGRESSION OF CURRENT DAKOTAS
ELEVATED CONVECTION WHICH MAY BEGIN REACHING NW TAF SITES AROUND
08Z. FOR NOW HAVE VCSH/-SHRA WORDING AND GENERALLY STILL VFR
ALTHOUGH VSBYS OR CIGS MAY BRIEFLY TOUCH MVFR AS WELL. BETTER
CHANCES ACROSS SERN SITES /KDSM/KALO/KOTM/. NO THUNDER MENTION AS
OF YET UNTIL TIMING AND LOCATION CONFIDENCE INCREASES. WINDS WILL
PICK UP INTO THE MORNING AND INCREASE FURTHER WITH FROPA SUN.
GUSTS WILL OFTEN EXCEED 30KTS BY THEN...REACHING WELL INTO THE 30S
NORTH AND WEST UNTIL DIURNAL LOSS OF MIXING BY EARLY EVENING.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
WIND ADVISORY FROM 10 AM THIS MORNING TO 6 PM CDT THIS EVENING
FOR CALHOUN-CARROLL-CERRO GORDO-CRAWFORD-EMMET-HANCOCK-HUMBOLDT-
KOSSUTH-PALO ALTO-POCAHONTAS-SAC-WEBSTER-WINNEBAGO-WORTH-WRIGHT.

&&

$$

SHORT TERM...SKOW
LONG TERM...ZOGG
AVIATION...SMALL



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