Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 261803
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
103 PM CDT SUN JUL 26 2015

.SHORT TERM /TODAY/...
ISSUED AT 324 AM CDT SUN JUL 26 2015

CURRENT CONVECTION IS DRIVEN BY A SHORTWAVE PUSHING ACROSS
NORTHWEST IOWA AND EXPECTED TO DROP SOUTHEAST THROUGH 18Z. THERE
IS A WEAK INSTABILITY AXIS RUNNING NW TO SE ACROSS IOWA IN WHICH I
BELIEVE STORMS WILL FOLLOW AS THEY MAKE THEIR WAY ACROSS THE CWA
THIS MORNING. STRONGEST STORMS WILL BE ALONG THE SOUTHERN
PERIPHERY OF THE MCS WHERE DCAPE VALUES ARE BETWEEN 1200 AND 1300
J/KG. AT THIS TIME THE LOW LEVEL JET AND BEST MOISTURE CONVERGENCE
REMAINS TO OUR WEST SO IF ADDITIONAL STORMS FIRE IN THE NEXT FEW
HOURS IN THIS AREA THEY COULD AFFECT WESTERN INTO SOUTHERN
PORTIONS OF CENTRAL IOWA LATER TODAY AND SOME HI-RES MODELS ARE
SUGGESTING THIS BUT I THINK THE OVERALL TREND IS SUGGESTING THIS
WILL CONTINUE TO PUSH EAST AND WEAKEN THROUGH THE MORNING.

FOR THIS AFTERNOON INSTABILITY INCREASES AS A WARM FRONT TRIES TO
PUSH BACK NORTH ACROSS THE STATE BUT WITH UPPER LEVEL RIDGING
TRYING TO RE-ESTABLISH ITSELF OVER EASTERN NEBRASKA THE AMOUNT OF
CONVECTION WE SEE THIS AFTERNOON MAY BE RELEGATED A LITTLE FURTHER
EAST...WHERE THE UPPER FLOW IS MORE NORTHWESTERLY AND THE
INSTABILITY GRADIENT IS GREATEST. THERE IS ALSO SOME WEAK FORCING
IN THIS AREA LATER TODAY AND THIS EVENING HOWEVER DEEP SHEAR IS
LACKING. IF STORMS DO DEVELOP IN THE INSTABILITY GRADIENT I AM NOT
LOOKING FOR ANYTHING SEVERE. TEMPS WILL BE A COUPLE DEGREES COOLER
TODAY EXCEPT FOR MAYBE THE SOUTHWEST.

.LONG TERM /TONIGHT THROUGH FRIDAY/...
ISSUED AT 324 AM CDT SUN JUL 26 2015

FROM TONIGHT THROUGH MONDAY EVENING A DEEP LAYER RIDGE WILL
CONTINUE TO BUILD TO THE SOUTH AND SOUTHEAST...EVEN AS AN UPPER
TROUGH SWINGS ACROSS THE NORTHWEST U.S. AND EJECTS ONE OR MORE
SUBTLE SHORTWAVE IMPULSES OVER IOWA AND THE MIDWEST. VARIOUS
PROGNOSTIC MODELS ATTEMPT TO SPIT OUT LIGHT AND SPOTTY QPF DURING
THIS TIME FRAME...HOWEVER THERE IS A LACK OF ANY REAL ORGANIZED
FORCING AND IT APPEARS THAT ANY EFFECT FROM THE AFOREMENTIONED
SHORTWAVE/S WILL PRIMARILY PASS TO OUR NORTHWEST. AS A RESULT WE
ARE LEFT WITH A SCENARIO WHERE A FEW SHOWERS AND STORMS ARE
POSSIBLE FROM TIME TO TIME BUT SHOULD BE OF VERY LIMITED EXTENT.
RATHER THAN BLANKET THE ENTIRE FORECAST AREA WITH SLIGHT CHANCE
POPS...HAVE OPTED TO GO WITH A NEARLY DRY FORECAST FOR
TONIGHT/MONDAY AND WAIT FOR SHORT TERM UPDATES TO IDENTIFY WHERE
AREAS OF ISOLATED STORMS MAY BECOME MORE LIKELY. AT ANY RATE GIVEN
THE LACK OF LARGE SCALE FORCING OR DEEP LAYER SHEAR SEVERE WEATHER
IS NOT ANTICIPATED DURING THIS TIME.

BY MONDAY NIGHT THE UPPER TROUGH TO OUR NORTHWEST WILL CROSS THE
NORTHERN ROCKIES...THEN SUBSEQUENTLY CLOSE OFF AND INTENSIFY AS IT
MOVES EAST NORTHEAST OVER SOUTH CENTRAL CANADA ON TUESDAY AND
TUESDAY NIGHT. INITIALLY A BROAD AREA OF ATMOSPHERIC LIFT WILL
ACCOMPANY THE FORWARD FLANK OF THE APPROACHING TROUGH...COINCIDING
WITH A NOCTURNALLY FAVORED TIME FOR STORMS TO DEVELOP WITHIN A
LLJ/WARM AIR ADVECTION REGIME ON MONDAY NIGHT. HOWEVER...THE BULK
OF THIS PRECIP SHOULD PASS BY TO OUR NORTHWEST AND HAVE HELD LOW
POPS TO OUR NORTHERN COUNTIES AS A RESULT. ON TUESDAY AND TUESDAY
NIGHT A SURFACE COOL FRONT ATTENDANT TO THE DEVELOPING UPPER
CYCLONE WILL THEN SAG SOUTHEASTWARD ACROSS IOWA. THE FRONTAL
PASSAGE SHOULD HELP TO GENERATE MORE THUNDERSTORMS...PARTICULARLY
AROUND TUESDAY EVENING AS PEAK HEATING ENABLES CONVECTIVE
TEMPERATURES TO BE REACHED. ON TUESDAY AFTERNOON THE CUMULATIVE
EFFECT OF SEVERAL DAYS OF GRADUAL WARMING...ALONG WITH MOISTURE
POOLING TOWARD THE APPROACHING BOUNDARY...SHOULD PUSH HEAT INDEX
VALUES UP TO NEAR ADVISORY CRITERIA EVEN IN THE ABSENCE OF ROBUST
LOW LEVEL MIXING. AN ADVISORY MAY THEREFORE BE WARRANTED AT SOME
POINT...BUT FORECAST HEAT INDEX VALUES ARE MARGINAL AND THERE
REMAINS UNCERTAINTY REGARDING THE AMOUNT OF CONVECTIVE CLOUD
DEBRIS THAT MAY BE PRESENT ESPECIALLY ON TUESDAY MORNING. AT ANY
RATE...AS STORMS DEVELOP ALONG THE FRONT AROUND TUESDAY EVENING
THEY SHOULD ASSIST IN ITS SOUTHEASTWARD MOVEMENT...CLEARING DOWN
INTO MISSOURI BY WEDNESDAY MORNING. DEEP LAYER SHEAR WILL BE
LIMITED IN THE FRONTAL/PRE-FRONTAL ZONE PARTICULARLY GIVEN THE
WEAK LOW LEVEL FLOW...HOWEVER STRONG INSTABILITY WILL PROVIDE AT
LEAST A MARGINAL THREAT OF SEVERE WEATHER WHICH IS WELL OUTLINED
IN SPC PRODUCTS.

FROM WEDNESDAY THROUGH FRIDAY THE WEATHER WILL TURN RELATIVELY
COOLER AND CERTAINLY MORE COMFORTABLE...AS A SURFACE HIGH PRESSURE
SYSTEM SLIDES ACROSS IOWA AND MISSOURI BENEATH NORTHWESTERLY FLOW
ALOFT WHICH WILL PERSIST THROUGH THE REMAINDER OF THE FORECAST
PERIOD. BY NEXT WEEKEND GRADUAL DESTABILIZATION AND A SERIES OF
SHORTWAVES MOVING THROUGH THE NORTHWESTERLY FLOW WILL RESULT IN A
RETURN OF THUNDERSTORM CHANCES TO THE FORECAST...HOWEVER ANY
CONFIDENCE IN DETAILS OF THIS SCENARIO IS VERY LOW AT THIS POINT.
TEMPERATURES SHOULD ALSO GRADUALLY MODERATE LATER IN THE
WEEK...UNLESS CHECKED BY CONVECTIVE EFFECTS.

&&

.AVIATION...26/18Z
ISSUED AT 100 PM CDT SUN JUL 26 2015

INITIAL CONCERN IS PEAK HEATING CONVECTION FOLLOWED BY RADIATION
FOG POTENTIAL OVERNIGHT. WATCHING AREA OF CONVERGENCE NEAR
KMCW/KALO THAT MAY DEVELOP ISOLATED SHOWERS AND STORMS INTO THE
EARLY EVENING. CONFIDENCE THAT ANY OF THESE WOULD IMPACT A
PARTICULAR SITE IS LOW HOWEVER SO ALL SITES VFR INTO THE EVENING.

WITH PERSISTENT CLOUDS INTO THE AFTERNOON IN THESE SAME
AREAS...LOW LEVEL RH WILL REMAIN HIGH LEADING TO FOG POTENTIAL
WITH CLEARING SKIES AND LIGHT WINDS. KMCW TEMP/DEWPOINT SPREAD IS
STILL ONLY 5F AT 18Z. HAVE MENTIONED VSBYS DOWN TO MVFR/IFR IN
THESE AREAS...ROUGHLY 09-15Z. SOME POTENTIAL FOR THIS TO OCCUR
EARLIER OR EVEN DOWN TO LIFR...BUT WILL HOLD OFF ON THAT UNTIL
CONFIDENCE INCREASES.

&&

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

&&

$$

SHORT TERM...FAB
LONG TERM...LEE
AVIATION...SMALL



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