Area Forecast Discussion
Issued by NWS Aberdeen, SD

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FXUS63 KABR 240627 AAB

Area Forecast Discussion...UPDATED
National Weather Service Aberdeen SD
1227 AM CST Wed Jan 24 2018

Issued at 903 PM CST Tue Jan 23 2018

Have slowed down the onset of fog, and shifted it farther east, given
the slow movement of the upstream low clouds and fog and model
guidance. Otherwise, few additional changes were needed to the
ongoing forecast.


.SHORT TERM...(This Evening through Wednesday Afternoon)
Issued at 317 PM CST Tue Jan 23 2018

The forecast challenge tonight into Wednesday will be whether or not
fog and/or low stratus forms over the CWA for a third consecutive
night, and its impacts on temperature.

Currently, temperatures are holding in the 20s across the eastern
third of the CWA, where low stratus clouds and patches of fog
persist. The sun is shining through some high clouds across the
western two-thirds of the CWA, where temperatures are warming into
the 30s and 40s.

The high clouds are associated with an upper level wave working
through the region. Once that wave/cloudiness moves southeast of the
region, another clear sky night on light (albeit westerly component)
winds will be in play. And, there will probably be adequate low
level moisture still around tonight in the nocturnal boundary layer
to aide in more fog or low stratus development, especially within
and between the Missouri and James River valleys and up on the
Prairie Coteau. East of the Coteau may end up being more stratus
than fog. Since there is support from the RAP in BUFKIT, along with
several other numerical guidance data sets, opted to stick with fog
mention in the weather grids for tonight into the first half of the
day Wednesday. Similar to last night, temperatures should not drop
off much more than perhaps 5 to 10 degrees underneath cloudy areas
tonight. Clear areas may be able to drop closer to 15 to 20 degrees
before stabilizing in fog/stratus environment, with a slow
temperature rebound expected on Wednesday, if fog/stratus tonight
pans out.

.LONG TERM...(Wednesday Evening through Tuesday)
Issued at 317 PM CST Tue Jan 23 2018

When the period opens Wednesday night, a large upper level ridge of
high pressure is beginning to build into the region. By Friday
morning, the upper ridge has shifted east and is being replaced by a
large/broad upper level trough of low pressure. By Sunday morning,
we`re right back where we started when the period opened, with
another upper level ridge trying to build into the region from the
west. However, the 12Z GSM deterministic solutions all agree today
on this next upper level ridge being suppressed while it works into
the middle of the country and for a brief period heading into day 7,
there could actually be nearly zonal flow over the region.

In this continued progressive pattern of ridge/trof/nearly zonal
flow, the forecast is still basically a dry forecast. The warming
trend still shows up in the deterministic low level thermal progs as
well as ensemble guidance through Friday. The onset of the broad
longwave upper trof still progs a cold frontal passage through the
CWA late in the week, with temperatures cooling back down to near,
or perhaps just below, climo normal for the weekend. The suppressed
ridge/nearly zonal flow set-up for early next week potentially
ushers a quick turn around back into some well above climo normal
temperatures by Tuesday.


.AVIATION...(For the 06Z TAFS through 06Z Wednesday Night)
Issued at 1223 AM CST Wed Jan 24 2018

Focus continues to be on the low stratus and fog that has moved
into northeast SD. KABR has already fallen into LIFR and expect
that to occur at KATY over the next hour or so. LIFR-VLIFR
conditions should continue through the night at those locations
and will improve during the morning hours as low level winds
increase out of the southwest and push the clouds/fog northeast.
For the other two sites, expect primarily VFR conditions, although
KMBG may see some MVFR vsbys on the western edge of the main area
of fog.




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