Area Forecast Discussion
Issued by NWS Aberdeen, SD

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FXUS63 KABR 161738

Area Forecast Discussion
National Weather Service Aberdeen SD
1238 PM CDT Wed May 16 2018

Issued at 1232 PM CDT Wed May 16 2018

Aviation discussion updated below for 18z TAFs.

UPDATE Issued at 1037 AM CDT Wed May 16 2018

Stronger winds around 925mb have mixed down to the surface this
morning, leading to sustained winds above 10 mph with gusts to
around 20 mph across parts of eastern SD and western MN. These
winds should lessen by noon as winds aloft weaken. Diurnal cumulus
clouds should develop this afternoon across the area, and some
isolated pulse thunderstorms still appear possible mainly across
south central SD as just enough instability develops.


.SHORT TERM...(Today and Tonight)
Issued at 354 AM CDT Wed May 16 2018

An isolated elevated convective cell remains over Lyman county this
AM, the lone straggler the result of a weak 500mb trough over
central SD/NE and warm advection in the low/mid levels. Anticipate
this will weaken with daytime heating in the mid morning hours as
with most nocturnal convection. Focus shifts to the morning model
soundings which show a deep warm/dry layer above a steep inversion.
Dewpoints should mix out, though there is a little more moisture in
that mixed layer to the south, meaning there will be a gradient
across the area of about 10 to 15F by the afternoon. Winds in
profiles mix out to 10kts or less, although there is a little bit
higher values in the NBL to mix out early across the far east into
western Minnesota. The little bit of a westerly component will also
help mix out the James valley and down wind of the Coteau earlier in
the day. Mixing between 700 and 800mb yields low to mid/upper 80s,
so will stick with the previous forecast highs. Afternoon convection
shows up in several CAMS, mainly in central SD where profiles show
bases around 6-7kft and upwards of 1000j/kg skinny CAPE. Winds in
the profile are also weak so isolated non-severe pulse type
convection is the thinking. Warm advection at 700mb and a weak 500mb
wave in southwest flow aloft will support isolated elevated
convection tonight which is also depicted in several CAMS. Most of
this will form on the nose of a 20 to 30kt low level jet. There will
be a noticeable gradient in MUCAPE, from around 1000j/kg in the west
to nothing in the east. Given weak shear aloft, it doesn`t appear
that there is a threat for severe weather, so this CAPE will
probably determine intensity of any precipitation. The low level
gradient tightens and we will see a slight increase in
southeasterly winds as well as low level humidity.

.LONG TERM...(Thursday through Tuesday)
Issued at 354 AM CDT Wed May 16 2018

Despite the "hybrid" Rex-Block split flow pattern persistence across
the western third/half of the CONUS throughout the extended portion
of the forecast, the pattern could be described as "active". About
the only timeframe in the extended when there aren`t chances of rain
is Sunday through Monday while the western CONUS reloads with more
upper level low pressure energy (beneath the upper level ridge
persistently parked over the Pac NW and the western third of

But this discussion will place a little bit more of the focus on
Thursday through Friday night. Based off the NAEFS/GEFS Ensemble
S.A. table, p-wats and spec humidity values and qpf amounts are all
2 or more standard deviations above mid-May climo normal, leading to
a little bit of concern that convection that gets going Thursday
night and then again later on Friday over the CWA could produce
heavy rainfall. The relatively slow-moving nature of convection in
this not-so-impressive deep-layer shear environment, but plentiful
CAPE/instability regime would support multi-cellular storm-mode and
perhaps a low probability potential Friday afternoon/night for
storms to train along the synoptic scale cold frontal boundary
forecast to be in play at that time. WPC`s Day 2 and Day 3 Excessive
Rainfall Outlooks include parts of this CWA in a marginal risk for
exceeding flash flood guidance. Given how dry sub-soil conditions
are, not all that concerned about flooding potential, should storms
end up training/producing high rainfall rates. Worth noting, though.
"Heavy rainfall" seems possible with convection Thursday night
through Friday night.

Temperatures are well above normal Thursday into Friday out ahead of
the synoptic scale cold front progged to move through the region on
Friday into Friday night. Breezy, cool and dry conditions are
forecast to develop into Saturday, with cool and dry conditions
persisting on Sunday while a broad Canada-sourced surface high
pressure system is centered over the northern plains. Temperatures
Saturday and Sunday, based off deterministic 00Z modeled low level
thermal progs and the NAEFS 850hpa S.A. ensemble table output,
should be generally below climo normal, but quickly begin a rebound
back to near to perhaps a few degrees above normal heading into the
first half of next week.


.AVIATION...(For the 18Z TAFS through 18Z Thursday Afternoon)
Issued at 1232 PM CDT Wed May 16 2018

VFR conditions are expected through the period. Only low chances
for thunderstorms until later in the day/evening on Thursday.




SHORT TERM...Connelly
AVIATION...Lueck is the U.S. government's official web portal to all federal, state and local government web resources and services.