Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 152324

Area Forecast Discussion
National Weather Service Sioux Falls SD
624 PM CDT Tue May 15 2018

.SHORT TERM...(This evening through Wednesday)
Issued at 311 PM CDT Tue May 15 2018

Will leave a mention of isolated to scattered showers and TSRA far
our far western zones into the very early evening hours. An axis of
800-750mb moisture exists in central SD streaming northward along an
area of warm air advection with convective temperatures reached.
Otherwise the night will be quiet. One thing to consider however was
the potential for fog in northwest IA again. The surface ridge will
ooze slowly southeastward into Iowa producing a dead flow of air in
that area. There are a lot of indications that fog will not develop
with hires NAM 950mb RH field progged to be much less early
Wednesday than it was for early Tuesday morning. In addition, the
various visibility values in short range models and ensembles are
not wild about the idea of fog in our forecast area. So for now,
left out the mention of it but this may need updated for northwest
IA if the surface dew point depressions once again get small this
evening. Wednesday looks like another seasonably pleasant day. Added
pops to our far western zones for the afternoon hours for virtually
the same thing going on as today. Moisture exists in the 800-700
layer coupled with a mid level short wave near 700mb and a
convective temperature which will likely be reached. Like today,
with 700mb temperatures only about +4C, it just does not take much
for a few showers or TSRA to develop.

Concerning temperatures, lowered some low temperatures tonight below
consensus guidance values across northwest IA and the southern
sections of southwest MN where the winds will be the lightest. That
put these locations in the upper 40s to lower 50s, with 50s
elsewhere. Wednesday`s highs still look to be in the lower 80s.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 311 PM CDT Tue May 15 2018

On Wednesday night, the aforementioned mid level short wave lifts
ever so slowly northeastward through central SD with weak 700mb
flow, coupled with a low level jet that is not overly strong.
Therefore only small pops are warranted for locations west of
Interstate 29 closest to this wave. There may be a few lingering
showers or small TSRA for basically the same locations Thursday
morning. But overall, Thursday is trending drier than previous model
runs as the previously advertised short wave riding eastward in ND
does not currently appear to be materializing. Therefore the short
wave moving through central SD just continues to move northeastward
through the day on Thursday and largely does not impact our area by
Thursday afternoon. Thursday`s highs look similar to Wednesday with
another seasonably warm day in store, as highs should be in the
lower to mid 80s.

On Thursday night and Friday, large upper trough of low pressure is
still forecast to move eastward into the intermountain west. The
various models are in good agreement in showing the first short wave
ejecting northeastward from this parent upper low across the western
plains Thursday evening, and into central NE and central SD late
Thursday night. This wave then ushers eastward into southwest MN and
northwest IA by late Friday. The bulk of the dynamics and 850mb
thetae advection steer generally northeastward as it rides up and
over an upper ridge centered over the eastern portions of the
southern plains. Therefore decent pops are warranted except for
perhaps in northwest IA where the short wave will have the least
influence. Therefore only slight chance to lower end chance pops are
warranted in our southeast zones on Friday.

This wave exits Friday evening but is rapidly replaced by a second
and stronger short wave which moves into central NE and southwest SD
Friday evening. This second wave then moves into the forecast area
late Friday night and is associated with a wind shift as an upper
trough of low pressure swings through southern Canada. High pops are
still warranted Friday night, lingering into Saturday morning at
least in our eastern zones where the wave is coinciding with the
wind shift. There will likely be pockets of heavy rain late Friday
night and Saturday morning, but severe weather is a bit more
problematic. Currently, the GFS soundings are not showing a ton of
instability. With mid levels that are still cool, +2 to +6C at 700mb
from north to south, the profile is not capped and therefore the
rain and TSRA has no trouble developing and thus not able to store
up positive energy. The directional shear ahead of the advancing
wind shift is highly veering, suggesting buoyancy, and the low level
jet is strong, but the mid level flow is quite weak. Therefore not
convinced there will be a lot of severe TSRA around.

The third and final short wave, which is really the remnants of the
parent upper low at that point, then moves eastward primarily
affecting our southern zones Saturday afternoon and evening. The
flow begins to get cutoff by the aforementioned Canadian trough
swinging southeastward into the western Great Lakes. As anticipated,
Sunday is now dry with all pops eliminated, and also not much in the
way of rain expected late Saturday night. Behind the cold front,
temperatures will be much cooler this weekend, below normal on
Saturday under clouds and rain, and a little bit below normal on
Sunday. Warmer temperatures will follow early next week.


.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 624 PM CDT Tue May 15 2018

VFR conditions should prevail with some redevelopment of diurnal
cumulus again on Wednesday.  There may be some shallow fog around
northwest IA and far southeast SD river valleys, but not expected
to have a great impact on visibility, thus have kept a few early
morning hours of shallow fog mention at KSUX.




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