Scientific Forecaster Discussion
fxus63 karx 201152
Area forecast discussion
National Weather Service La Crosse WI
552 am CST Wed Nov 20 2019
Short term...(today through thursday)
issued at 130 am CST Wed Nov 20 2019
Remarkable model agreement over the past several days, both within
and between the themselves, with the evolution and track of the low
pressure system to impact the region Wed night-Thu.
Forcing: an upper level shortwave trough still slated to kick
northeast out of the Desert Southwest tonight, spinning through the
region late tonight/Thu morning. Associated sfc low moves over the
local forecast area by 12z Thu, moving into eastern/northeast WI by
18z. Good low level thermodynamics lead the system across the area,
with some additional lift from a coupled 300 mb overnight Wed. No
problem with saturation for pcpn production.
What falls: northwest-southeast running time/height x-sections along
with BUFKIT soundings continue to show ample warming in the low
levels, and relatively deep: above zero through around 700 mb. This
makes rain the pcpn type for the bulk of the quantitative precipitation forecast. Some cooling
starts to work in as the system exits Thu, and also some potential
for loss of ice in the cloud. Could see a brief period of a wintry
mix on the tail end of the system.
How much: very wet air mass of the end of November with pws from 1
to 1 1/4 inches and warm cloud depths around 2.5 kft. Naefs precipitable water
anomalies still from +2 to +3 locally during this time. Considering
the relatively strong dynamics coupled with the abnormally moist
airmass, seeing in excess of an inch for some locations seems more
likely than not. Snow or any potential icing amounts look minimal at
best right now and mostly confined to the far west/north.
Winds: in addition to the soggy conditions, winds are going to be on
the increase, especially Post the low for Thu/Thu night. While
mixing looks fairly shallow at this time (limiting how gusty it
could get), the tightening pressure gradient suggests 20-25 mph
sustained for the open/unsheltered areas west of the Mississippi
Long term...(friday through tuesday)
issued at 130 am CST Wed Nov 20 2019
The 20.00z GFS and 19.12z ec still favoring dry conditions through
the weekend into early next week, but starting to show some
potential for a storm system that could sweep in from the southwest
starting Tue. Current progs (take with a grain of salt this far out)
would track the storm along a similar path to what is expected from
the system for Wed night-Thu. Areas/bands of pcpn would accompany
the storm, but this time there could be colder air to deal
with...and snow (at least on the north/northwest side of the low -
deformation region). Again - needs to be stressed - this is a week
out and lots can (and will likely) change between then and now, but
considering we will be approaching the Thanksgiving Holiday by then
with associated big travel days, it's a storm that deserves to be
closely monitored. And that said, the 20.00z ec is weaker and more
south with that potential storm. Plus, it should be noted that most
ensemble members of the gefs don't support the operational GFS.
Sooo...stay tuned, but don't get overly excited yet.
Temps will take a dive for Fri Post the Wed/Thu with colder air
dropping in from Canada. Should start to moderate then for the
weekend and the early part of the new week.
Aviation...(for the 12z tafs through 12z Thursday morning)
issued at 516 am CST Wed Nov 20 2019
Patchy dense fog and low stratus will gradually lift this
morning. Conditions are expected to improve to VFR by early
afternoon. Overnight tonight, MVFR will return with lower ceilings
and reduced visibility in rain associated with a broad low
pressure system lifting across the Midwest. Low level wind shear
may also become a concern tonight due to strong winds above the
surface in the warm sector of the system.
issued at 130 am CST Tue Nov 19 2019
Rainfall in excess of 1 inch looking likely for parts of the area
from Wed night into Thu morning - the bulk falling during the
overnight hours Wed.
The shallow frost in the ground has started to thaw but the top
layers of soil remain wet and won't absorb much. Much of the rain
will therefore go into runoff into area waterways, resulting in
rises on some rivers and streams. Within bank rises are currently
expected, but cannot rule out minor flooding in some areas. If
rainfall amounts increase, minor flooding will become more likely.
Monitor later forecasts and river levels closely this week.
WI...dense fog advisory until 9 am CST this morning for wiz017-029-