Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 182051

Area Forecast Discussion
National Weather Service Des Moines IA
351 PM CDT Tue Jul 18 2017

.SHORT TERM.../Tonight/
Issued at 351 PM CDT Tue Jul 18 2017

Weak area of low pressure situated off the southwest with an
extended lobe stretching off to the northeast through northern IA
into SE MN and central WI. Have some storms forming along the
boundary across northern IA and expect that trend to continue into
the early evening hours as weak energy pushes through MN aloft. CAPE
decent around 1500 J/kg, but effective shear only around 20 kts. May
get a stronger storm into the evening as shear may increase a
little, but overall severe threat is expected to be lower across
northern IA with a greater chance off to the north and east. Some
other pop-up storms forming across the southeast where decaying MCS
lingering from earlier this morning. This should continue to move
east into early evening with any activity diminishing. Overall
temperatures to remain warm overnight with WAA aloft with the heat
building in for the next few days.

.LONG TERM.../Wednesday through Tuesday/
Issued at 351 PM CDT Tue Jul 18 2017

Two main things to look at initially in the long term with the heat
during much of the period and then thunderstorm chances Wednesday
night. H85 temperatures to climb into the mid 20s celsius Wednesday
with a broad upper ridge in place aloft. Main flow of westerlies to
be situated just to the north. This will allow the heat to build as
an area of sfc high pressure dominates much of the area through the
remainder of the week into the weekend. Moisture to flow off the
Gulf and into IA with dewpoints to be around 70 and into the mid
70s. Overall temperature/dewpoint combination still has some
uncertainty with potential for overnight convection both Wednesday
night and Thursday night into Friday the temps. Also mixing could
cause dewpoints to be a bit lower than anticipated at times as well.
Tried to mitigate these factors some with the forecast to give the
better look at expected conditions. Therefore upgraded the southern
three tiers to a warning where heat index values should exceed 105
degrees Wed/Thu/Fri and be near 105 Saturday. Went with an advisory
for points further north that should reach 105 Thu/Fri and above 100
Sat and Sun. The big issue will also be the prolonged nature of the
heat with it continuing for 4 days with little relief at night as
temps only dropping into the mid to upper 70s at night. Some
adjustments may be needed through the period to spatial extent
depending on convective activity the next few nights, etc. Ridging to
dampen into the weekend as a cool front associated with an upper
trough moving across south central Canada drops through the state.
This should allow for a slight cool down into the end of the weekend
and early next week.

As for thunderstorm chances, Wednesday night into early Thursday
looking like the best chance for some rain across portions of the
area into the weekend. Area of low pressure off to the southwest to
lift north/northeast with a boundary in place across the Dakotas/MN.
Activity is expected to fire in the vicinity of the boundary across
the Dakotas/MN late Wednesday and drop down southeastward along the
edge of the heat dome into northern IA/eastern IA late Wednesday
night. Strong LLJ oriented into southern MN/northern IA by late
night, with strong shear expect the thunderstorm complex to
potentially have a decent wind threat as the MCS dives
southeastward. Greatest chances will be around the kMCW area where
have gone with likely pops. Otherwise 12Z NAM somewhat the odd model
out into late Thursday night/Friday trying to develop another
thunderstorm complex through the state with storm potential
lingering into Friday. GFS/EC keep the boundary further north
keeping any activity likely over southern MN/far northern IA. If the
NAM were to verify however, Friday temperatures would likely be too
warm and could disrupt the prolonged period of heat.


.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 128 PM CDT Tue Jul 18 2017

Some high clouds around with southwesterly winds in place. Winds
to become more southerly to southeasterly through the period. Low
confidence on chances for storms across the north, so highlighted
with a VCTS for now. However still some possibility for
development near KFOD and KMCW this afternoon/early evening with a
few small cells already across northwest into north central IA.


Heat Advisory from noon Wednesday to 8 PM CDT Saturday for

Excessive Heat Warning from noon Wednesday to 8 PM CDT Saturday
for IAZ070>075-081>086-092>097.



SHORT TERM...Beerends
LONG TERM...Beerends
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