FXUS63 KDMX 060435

Area Forecast Discussion
National Weather Service Des Moines IA
1131 PM CDT Fri Jun 5 2020

.DISCUSSION.../Tonight through Friday/
Issued at 258 PM CDT Fri Jun 5 2020

Synoptic Overview:

Short-wave trough with 70 kt H5 jet streak is moving from the upper
Midwest into the northern Great Lakes Region as of this morning.
Weak, broad ridging has been occurring over the Northern Rockies, and
the axis is sliding eastward toward the northern Plains with another
trough off the NW Pacific Coast. Surface low pressure is also
pushing eastward, with surface high pressure over the northern
Plains strengthening with the AVA under the ridge setup. This has
been slowly pushing a boundary southeast across the Missouri River
Valley today.


Strong WAA over the Plains will result in amplification of the H5
ridge that is shifting eastward across the central and north central
CONUS. This afternoon, that will result in mid-level flow briefly
being northwesterly. On the backside of the surface low over the
Great Lakes and with surface high pressure over the northern Plains,
near surface flow will also be generally northwesterly. As a result,
this will push the boundary southeastward throughout this afternoon.
Since 12z, CAM guidance has backed away from placing light showers
and weak thunderstorms in southwest Iowa later this afternoon and
early evening. Exploring the soundings of the HRRR and RAP, the
solutions indicate rather dry air between 850 mb and 650 mb, which
certainly would suppress shower activity. However, lapse rates in
the low to mid levels are somewhat steep, and with clouds clearing
and plenty of insolation will likely be realized. This may be just
enough for a few parcels to rise and result in a few weak showers.
Thus, have slightly decreased POPs for later this afternoon and
evening, but have not gone completely dry. As for temperatures this
afternoon, expecting a decent recovery as clouds clear in plenty of
time for peak heating. Northern and northwest Iowa will remain
cooler on the north side of the boundary, with temperatures in the
lower 80s. Central and southern Iowa will reach the mid to upper
80s. South of the boundary, with dewpoints in the upper 60s to lower
70s, heat index values will be in the lower 90s. This will make for
an overall muggy day across central and southern Iowa.

Saturday and Sunday:

Saturday the synoptic pattern will begin to get interesting with
the northward progression of what is now Tropical Depression
Cristobal, moving into the open Gulf of Mexico off the Yucatan
Peninsula. The axis of the amplified H5 ridge will attempt to slide
east, being set in motion by a negatively titled short-wave trough
in the southwest CONUS and an Aleutian low centered off the NW
Pacific Coast. As surface high pressure slides eastward with the
ridge axis advancement, low-level flow turns southerly which will
advect a warm-moist airmass back into the upper Midwest immediately
after seeing the passage of the weak cold front from Friday
afternoon. Afternoon high temperatures will recover back into the
upper 80s for western and southern portions of the forecast area.
Rain shower and thunderstorm chances Saturday will be highly
dependent on the role of Cristobal has in blocking the eastward
advancement of the ridge axis, as the short-wave trough quickly
ejects northeastward into the Plains with a 70 kt H5 jet streak.
This will provide a shot of CVA on the backside of the ridge and
tries to force sudden height falls over the upper Midwest. However,
12z GFS/NAM//ECMWF guidance currently depicts Cristobal halting any
eastward progress of the mid-latitude flow. Thus, this would keep
heights steady Saturday while forcing the quick ejecting short-wave
trough into the Canadian Prairie Provinces. This will certainly lead
to some kind of perturbation in the mid-level flow across the upper
Midwest, but 12z cycles of most guidance now keeps the better
forcing for precipitation in the eastern Dakotas and western
Minnesota. The GFS is only the model that attempts to have any kind
of light rain shower activity in northern Iowa. Most ensembles
members remain dry Saturday, thus will keep POPs and rain showers
chances extremely limited across northern Iowa. Sunday, the large
scale pattern remains roughly the same as Cristobal continues to
track inland. The short-wave trough axis continues to swing
northeastward into Canada. Another shot of weak CVA may reach the
northern extent of the forecast area, which could force out a light
shower. Thus have a brief period of low POPs north of U.S. Hwy. 18
going for Sunday. As with any extratropical cyclone, changes in the
track will greatly influence these limited precipitation chances.
The main weather story for the rest of the forecast area on Sunday
will be brisk winds, as surface-cyclone associated with the short-
wave tracks through the central Plains and increases the strength of
the pressure gradient. At this time, expecting wind gusts to remain
well below wind headline criteria.

Extended Forecast:

Warmer temperatures will continue through Monday while the H5 ridge
remains in place across the upper Midwest and southerly flow
continues to move in. Cristobal (or what is left of it) will bring
rain shower and thunderstorm chances Tuesday through Wednesday. The
12z cycle of the GFS and ECMWF are actually in decent agreement
about brining widespread rainfall to Iowa with this cyclone. The GFS
is just a tad faster, but overall both models track the center of
the cyclone along roughly the same track. Expect thick cloud cover
Tuesday through Wednesday, which accompanied by heavy rain showers
will help to reduce temperatures across the state thorugh the middle
and end of the next work week. QPF will need to be monitored closely
with rainfall associated with Cristobal. While several areas will be
able to handle a decent amount of rainfall, other areas may begin to
see flooding issues, and will also need to be concerned about river
levels. As QPF becomes more certain, will have further discussions
about hydrology concerns across the state.


.AVIATION.../For the 06Z TAFS through 06Z Saturday night/
Issued at 1131 PM CDT Fri Jun 5 2020

A weak disturbance combined with some moisture and lingering
instability is kicking off isolated storms. there is little to
support development so the storms pop up then fall apart. This is
making the TAF forecast difficult since the "hit and miss" nature of
the storms provides low confidence that they occur near a TAF site.
KFOD and KMCW appear to be most likely to have TSRA near the airport
so I felt VCTS was warranted. Once the disturbance passes towards
daybreak, VFR conditions will prevail with a southeasterly wind
around 10kts. Some higher gusts will occur in the afternoon.





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