Area Forecast Discussion
Issued by NWS Hastings, NE
FXUS63 KGID 191721
Area Forecast Discussion
National Weather Service Hastings NE
1221 PM CDT FRI AUG 19 2016
.SHORT TERM...(Today and tonight)
Issued at 314 AM CDT Fri Aug 19 2016
A cold front is making its way across the local area early this
morning. Expect this front to continue to slowly track southeast
through the afternoon hours, with shower and thunderstorm activity
becoming likely across much of the local area during the late
morning through afternoon hours. While most areas should see some
precipitation today...confidence in waning on the potential of
strong to severe thunderstorms with substantial rainfall...at
least across the tri cities.
While the 6Z run of the NAM still holds onto widespread
significant precipitation later today...the timing of the front is
obviously off as according to radar and the latest surface obs
this front is currently already centered across the heart of the
forecast area this morning. As a result...thinking is that the
NAM is way overdone in its QPF and thunderstorm coverage forecast
for today...with the more realistic solutions being the HRRR and
RAP...which have much less coverage across the tri-
cities...focusing the better chances for more significant
precipitation to our south and east where the better instability
resides. As a result...it is not surprising that the slight risk
for today has shifted southeast overnight.
Despite the decreasing confidence in the precipitation forecast
for today, confidence has increased for cooler afternoon temps
across most areas this afternoon as the front will be further
southeast than originally thought. As a result...opted to
decrease high temperatures severals degrees in most spots from the
inherited forecast, while maintaining the high chances for precip
as despite the less favorable location of the front, lower stratus
and some thunderstorm activity can still be expected behind this
front through the afternoon and evening hours as an approaching
upper level shortwave will aid in tapping the elevated instability
across the local area. This shortwave should clear the local area
overnight...with partially clearing skies expected by the early
morning hours Saturday. With these partially clearing skies and
strong cold air advection behind the front...expect refreshing
overnight low temperatures tonight, with most locations dropping
into the lower to middle 50s.
So in summary...still expect shower and thunderstorm activity
across the region today, but confidence in significant rainfall
totals and strong to severe thunderstorms has shifted
southeastward...likely closer to the passing cold front which
should be southeast of the tri cities by the afternoon hours.
This more progressive placement of the front will result in a
return to cooler than normal temperatures for this afternoon and
.LONG TERM...(Saturday through Thursday)
Issued at 314 AM CDT Fri Aug 19 2016
Aloft: An anomalously deep trof (-3 SD) will be overhead at daybreak
Sat. This trof will exit E of the region this weekend while the low
currently over the Aleutians drops into SW Canada Sun. Heights will
here on the Plains Sun-Mon from 576 to 588 with a ridge cresting
overhead Mon. The Canada low will cont E thru the Srn Prairies...
with a trof developing S into the Cntrl Plains Wed-Thu.
Several vort maxima will meander thru the Desert SW Sun-Mon. One or
more of these could eject onto the Plains Tue and Wed...ahead of the
approaching Nrn trof. This could enhance tstm activity.
Surface: Low pres will be over the Great Lakes Sat morning. The
associated cool front moving thru this morning will be well E and S
of the fcst area. Cool high pres will be over WY/CO. This high will
gradually drift across the area Sat night and lodge over the Ern USA
for much of the upcoming week. Meanwhile...cyclogenesis will occur
over Alberta/Sask as early as Sun...but it isn`t fcst to mature and
begin moving E until Mon night. As it heads E across Srn Canada Tue-
Wed...its cool front will surge S. Fropa should occur here Tue night
into Wed morning followed by another Canadian high.
Some daily details...
Sat: The fcst area will be in the cool sector of the Great Lakes
low. The day will start m/cloudy from the Tri-Cities N and E and
p/cloudy W and S. Decreasing clouds from W-E thru the day...but
still expect SCT stratocu to develop with the thermal trof overhead
for much of the day.
Unseasonably cool. The +10C 850 mb isotherm will be down to the
Neb/KS border. Full sun suggests low-mid 70s.
Wind: Confidence in the wind fcst is below average. GFS MOS has 12
kts at GRI in the morning. NAM MOS has 22 kts. That is a substantial
diff. Overlaying the last 2 runs of the EC/GFS/NAM/GEM and the SREF
mean show the diff in wind fcsts is due to uncertainty in the
position of the low. This uncertainty is also in the 12Z/18 EC
ensemble. Played the fcst conservatively. WPC noted in its 1249 am
Model Diagnostic Disc that the 00Z NAM was much deeper and slower
than the model consensus. Its 12Z/18 run was deep/slow as well.
Sun: WAA. Dry with rebounding temps back into the 80s.
Mon: Dry and warmer. Temps back to near normal (mid-upr 80s).
Breezy SW winds probably G30 mph.
Tue: Fcst has a 20-30% chance of tstms. Believe this is overdone.
Daytime hrs are probably dry...but the chance at night is legit.
Wed-Thu: Uncertainty. If one of the vort maxima coming out of the SW
is timed right...it could hang up the front and enhance tstm chances
along and N of it. But even the last 2 runs of the EC disagree on
this. Stay tuned...
.AVIATION...(For the 18Z KGRI/KEAR TAFS through 18Z Saturday afternoon)
Issued at 1217 PM CDT Fri Aug 19 2016
Primary concern for this TAF period lies in the next 6 hours, as
an upper level disturbance sliding through the region brings
continued thunderstorm chances to both terminal sites. Based on
what EAR reported with the first batch of storms and looking
upstream at obs, decided to insert a tempo mention of MVFR
conditions at both sites for a couple of hours. Left as VCTS
otherwise. Adjustments with time may be needed, will see how
precipitation evolves. As far as this evening/tonight goes with
the passage of the main upper trough, models continue to shy away
from either site being affected by precipitation, so kept mention
out, for now. Winds are expected to remain north-northwesterly
through the period.