Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 151743

Area Forecast Discussion
National Weather Service Des Moines IA
1243 PM CDT Thu Jun 15 2017

.SHORT TERM.../Today through Tonight/
Issued at 317 AM CDT Thu Jun 15 2017

Confidence: Medium to High

Somewhat complicated forecast through tonight...not so much for
afternoon high temperatures but for thunder chances this afternoon
and into tonight. Late evening surface analysis showed lingering
outflows/wind shifts over the southeast and north with small
chances in moisture characteristics. Through the day today warming
will occur with early morning fog possible then generally mostly
sunny through early afternoon. Highs will recover into the lower
90s over most of the area by late afternoon. By mid to late
afternoon convective elements will begin to occur from the
northeast toward the south as a weak upper level wave/height falls
move into the area. Will keep slight chance thunder northeast by
22-00z then building back south southwest into the evening hours
as weak warm air advection aloft begins to overrun the upper level
boundary/weak surface front. Toward 06-12z another minor impulse
at H700/500 will enter and then cross the forecast area through
16z. This collocated with the surface boundary and weak warm air
advection into the area should help set off thunderstorms into the
evening hours... increasing late into the night as H850 winds
increase to 20 kts across the region. Bulk wind shear increases
both in the afternoon with the first wave northeast as well as
between 00-06z in the southwest with values between 40 to 50kts.
Though there is some uncertainty regarding the coverage of looks like into the afternoon hours to evening today
coverage will be isolated with scattered to more widespread
coverage later in the evening/night as the stronger shortwave and
return warm air advection punches northeast into
southern/southwest Iowa. The main threats appear to be hail and
wind...with lessening instability toward 06z as instability
lessens. Lows tonight will remain quite warm with clouds...a warm
airmass from the day and continued light south winds through the
night resulting in lower 60s north to mid to upper 60s south.

.LONG TERM.../Friday through Wednesday/
Issued at 317 AM CDT Thu Jun 15 2017

The period from Friday through late Saturday continues to look
warm and active with multiple rounds of severe weather potential.
From Sunday through Wednesday we will reside beneath relatively
cooler and generally quiet west northwesterly flow, with a few
opportunities for additional rain but a much lower potential for
severe weather or widespread precipitation.

On Friday morning overnight convection will be just exiting our
southeastern counties, leaving a somewhat convectively overturned
airmass in its wake. However, rapid destabilization will occur
during the day and an effective warm front will likely become re-
established somewhere roughly around central or northern Iowa. A
subtle mid-level impulse moving quickly through brisk zonal flow
aloft will reach the South Dakota/Nebraska area by late Friday
afternoon and appears likely to generate intense convective
updrafts somewhere around eastern Nebraska and along the warm
front/dry line. Strong instability and moderate, but more than
sufficient, nearly uni-directional shear aloft will support rapid
upscale growth into a forward-propagating convective complex with
an attendant threat for strong/damaging winds. Given the strength
and orientation of the steering flow and the relatively weak low-
level winds, it is likely that if such a complex does develop, it
will pivot east southeastward into the stronger instability
across southern Iowa and into northern Missouri. SPC has outlined
much of our forecast area in a slight risk during this time frame,
with the potential for part of that area to be upgraded in
subsequent updates.

The Friday scenario will be repeated on Saturday, but with notable
differences that could result in a more significant, potentially
high-end severe weather event. A much stronger mid-level shortwave
trough will follow the same trajectory across South
Dakota/northern Nebraska in the late morning/afternoon, and
Minnesota/Iowa by early Saturday night. This system will exhibit
much stronger forcing for vertical ascent and wind fields aloft,
with 500 mb winds around 70 knots or so during the afternoon and
evening hours. There is some uncertainty regarding the southward
extent of, and degree of convective overturning due to, the Friday
night convection discussed above, and how far north significant
destabilization will be able to return during the day on Saturday.
The NAM provides a glimmer of hope here as the Saturday afternoon
instability pool is located over northern Missouri and far
southern Iowa. However, the bulk of model solutions bring it much
further north, across at least the southern half and perhaps most
of Iowa, by Saturday afternoon. While there is some inherent
uncertainty due to dependence on the preceding round(s) of
convection, there is at least a distinct possibility of a much
stronger forward-propagating MCS across portions of our forecast
area on Saturday afternoon, and given the strength and uni-
directional nature of the flow aloft, it would carry a high-end
wind threat perhaps even qualifying as the dreaded "D" word. This
potential will need to be watched very closely over the next
couple of days.


.AVIATION.../For the 18Z TAFS through 18Z Friday afternoon/
Issued at 1243 PM CDT Thu Jun 15 2017

Convective trends will be the primary aviation challenge through
the period. Isolated/scattered thunderstorms likely to develop
along a line from near KALO/KDSM late this afternoon then become
more focused southern Iowa including KOTM this evening. Outflow
boundaries and gusty winds will be possible in vicinity of storms.
Late tonight into Friday morning should be mostly quiet with
south to southwest winds at 5 to 15 kt and VFR conditions.




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