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FXUS63 KTOP 061127

Area Forecast Discussion
National Weather Service Topeka KS
527 AM CST Fri Dec 6 2019

...Update to aviation forecast discussion...

.SHORT TERM...(Today and tonight)
Issued at 215 AM CST Fri Dec 6 2019

Colder air streams southward today, with overcast skies gradually
clearing this afternoon.

A compact upper tropospheric shortwave trough embedded in a mean 
zonal west-northwest flow is departing the region this morning, 
with increasing subsidence throughout the day as broad ridging 
builds in its wake. Ongoing CAA behind the corresponding surface 
cold front tapers off by mid-afternoon as a 1030mb surface high 
translates across the north-central CONUS. Even with the 5-7 C 
drop in H850 temperatures between 00Z and 15Z today, highs should 
still approach average for this time of year. Low stratus 
migrating southward within this northwest flow regime is becoming 
established over the forecast area early this morning. These 
clouds look to linger through the morning, with the short-term 
model consensus being that the increasing subsidence results in 
the complete erosion of the low-level saturated layer by midday. 

There is some concern that the models are being too aggressive in
the breakup of the stratus given that the WAA does not commence 
until later tonight. The overall fetch of low clouds extends into 
central SD, with ongoing northerly flow at the time the clouds are
progged to dissipate. Given the strong deterministic and 
ensemble-based model consensus in this decay, have gone with 
clearing skies in the going forecast, but would not be surprised 
to see them linger longer into the day. With upper level ridging 
in full control overnight and weak WAA commencing, expecting clear
skies and lows to stabilizing in the middle 20s.

.LONG TERM...(Saturday through Thursday)
Issued at 215 AM CST Fri Dec 6 2019

The weekend features a return of above average temperatures 
before a strong push of colder air arrives early Monday.

The surface ridge axis slides east of the forecast area Saturday 
morning, allowing for increasing southerly winds and WAA to 
overspread the area. H850 temps increase to around +5 to +7 C 
Saturday afternoon with the mixing heights rising to around H925 
in the afternoon, yielding high temperatures in the low to mid 
50s. It is certainly possible that we mix slightly deeper and see 
highs push the mid to upper 50s in some areas. Increasing theta-e 
advection late Saturday night into Sunday morning will likely 
result in the development of a stratus deck by sunrise Sunday. The
deepest saturation is forecast to be along and east of a 
Manhattan to Marysville line and persist through the day on 
Sunday. These clouds make the high temperature forecast for Sunday
a bit less certain, but modest WAA and warm overnight lows 
Saturday night should ensure that we reach at least the low 50s, 
with the mid to even upper 50s within reach.

Attention then turns to a strong push of a continental arctic air
funneling southward as the upstream pattern amplifies. Over the 
weekend, a lobe of shortwave energy works southward along the 
western flank of the Polar Vortex anchored over Hudson Bay. A 
corresponding cold front surges southward Sunday night, blasting 
through the forecast area by sunrise on Monday. Transient periods 
of frontogenetical forcing within a weakly saturated profile may 
yield brief periods of flurries or sprinkles on Monday, but the 
overall threat for measurable precipitation is low. The cold (and 
dry) air lingers into midweek before southerly flow returns. 
Medium-range solutions begin diverging on the characteristics of 
the upper level ridge axis by midweek, yielding a 20-25 degree 
spread in the surface temperature forecast by Wednesday.


.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 525 AM CST Fri Dec 6 2019

Ongoing MVFR stratus/stratocumulus rapidly clears by midday, with
VFR conditions forecast for the remainder of the TAF period. 
Gusty northwest winds early this morning weaken this afternoon and
become light and variable overnight.




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