Scientific Forecaster Discussion
fxus63 ktop 061127
Area forecast discussion
National Weather Service Topeka Kansas
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 cold air advection behind the corresponding surface
cold front tapers off by mid-afternoon as a 1030mb surface high
translates across the north-central Continental U.S.. 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 warm air advection 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 warm air advection 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 warm air advection 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 warm air advection 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.