212
FXUS63 KARX 150854
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
255 AM CST Fri Nov 15 2024

.KEY MESSAGES...

- Mild weekend ahead. Breezy Saturday. Light rain chances Sat night,
mostly focused over WI (20-60%) but only minor accumulations (trace
to under 1/10")

- Complex weather scenario for next week presents a lot of
uncertainty. Rain is looking likely for Monday night into Tue
(current probabilities for over 1" upwards of 60% for southeast
MN/northeast IA). Colder air (albeit more seasonable) also expected
with greater uncertainties on snowfall possibilities and stronger
winds.


&&

.DISCUSSION...
Issued at 255 AM CST Fri Nov 15 2024

- MILD WEEKEND AHEAD: increasing light rain chances (20-60%), mostly
over WI Sat night. Amounts would be minor (trace to under 1/10")

The long range guidance has been steady with bringing an amplifying
upper level ridge across the region today, but with a fairly
progressive flow, it quickly exits east Sat allowing for shortwave
troughing to traipse west-east across northern MN/WI. Post this
system, another shortwave ridge works Sun night/Mon.

Milder air pools northward and promises a few days of above normal
temps - even with a cool frontal passage with the shortwave Sat
night. LREF/HREF has 80-100% chances for 50+ degree highs today/Sat,
with the higher probs shifting a bit more south/southeast Sun/Mon.
Roughly 10 degrees above the mid November normals.

As for rain chances, still a disjointed Fgen signal around the sfc
front for Sat night but models still paint a ribbon of northward
moving  850 mb moisture transport ahead of it. More stretching
southwest of the vorticity in latest NAM/GFS/EC, helping with lift
around that sfc boundary. Depth of saturation remains the main
drawback to pcpn production - but the models are showing signs of
trending up with moisture. The GFS in particular has been the
"driest" of models, keeping the deeper saturation north with the
parent shortwave, and thus produced little if any QPF along the
extent of the front. That has changed with the last 2 runs of the
GEFS with most of its members suggesting at least measurable pcpn.
THe EC has always been more enthusiastic with rain chances and
continues such. NAM is similar with suggesting rain. In most cases,
forcing will overcome any "perceived" lack of saturation - and
believe this is one of those cases. NBM has increased rain chances
(20-60% locally), mostly for western/northern WI, and these look
reasonable. Believe this will continue to trend toward a low
QPF/high pop scenario and rain chances will likely need to be
increased while sensible outcomes result in only trace to a few
hundreths for QPF.


- NEW WORK WEEK: periods of rain, colder air, potential for snow
and some wind. A lot going on, a lot of uncertainty.

First - be wary of buying into any particular model solution. There
are so many moving parts for next week and how various shortwaves
interact/move will have a considerable impact on what kind of pcpn
falls where, how much, and even the degree of cold that filters into
the region. There have already been swings back and forth within the
long range guidance and their ensemble suites - and expect more
adjustments as we move into next week. Highest confidences lies with
the likely rains for Monday night/Tue.

So, let`s take this piece by piece...

1) A shortwave trough still slated to lift northeast out of the
desert southwest Sunday night, spinning over the upper mississippi
river valley by 12z Tue. While there has been, and continues,
differences in where the GEFS and EPS slide an accompanying sfc low,
trends have taken the same path, same relatively location (over IA
by 12z Tue), and timing. A warm front will lift northward with the
low with fairly hefty west-east running low level warm advection
driving across the region Monday night. GFS suggest a 50 kt low
level jet will impinge on/across the warm front, with an
accompanying strong surge of 850 mb moisture transport. Expansive
area of rain will result, as soon as Monday afternoon, persisting
through Monday night, and shifting north Tue.

Rainfall could be relatively significant for this time of year. PWs
in the GFS push 1 1/4". NAEFS and EC PW anomalies are +2 to +3 for
PWs. EFIs for PWs are 0.7 to 0.8 with SOT of 1 just of to the west
(over southwest MN) for the 00z Tue-00z Wed period. This has been an
increase (considerable) over previous runs. All this points to an
unusually wet airmass for mid November with the potential for
relatively high amounts of rainfall (compared to climatology). The
GEFS has been more "wet" compared to the EC with its GEFS mean qpf
from 1 to 1 1/3" for this period. Shave a third of that off and its
closer to the EC. Grand Ensemble probabilities for 1"  or more
currently range from 60% in the southwest to 35% north of I-94. Of
note, the GEFS upper 10% of outcomes drops rainfall closer to 2".
The rain looks progressive and don`t anticipate any flooding
concerns at this time - but given the unusually wet airmass that
continues to be portrayed in both the GEFS/EPS suites - a close eye
needs to be kept on evolution/model trends as we move into next
week.


2) Another shortwave trough moves off the west coast to over the
southern plains by 00z Wed with latest deterministic runs of the GFS
and EC rotating the Mon night shortwave into this feature, and then
pushing the merged system over the OH/TN River valleys by 00z Thu.
Ensemble means in both members follow suit, but still some widely
dispersive members evident. If this bears out, a slug of colder
(albeit more seasonable) air drops in from Canada, but most of the
related pcpn chances could/would hold over northern/eastern WI - the
far western edges of the deformation region/westward hanging sfc
trough of the parent low. The NBM lays out low end pcpn chances for
most of the region in the Wed/Thu time period - indicative of that
model uncertainty and variety of potential outcomes. Will run with
those chances. Note - due to the colder air, some light snow enters
the forecast.


3) A third shortwave had been diving almost due south across the
plains moving into Thu/Fri, retrograding the eastward moving system
a bit, but also bringing a stronger slug of cold, Canadian air
across the region. 15.00z GFS runs takes this feature on more of a
southeast tact, slipping in and out of the region Fri/Fri night. The
same run of the EC has shortwave ridging building in, and keeps the
shortwave trough Canada as a result.

An interesting outcome for the EC as this solution has been one of
the WPC clusters over the past several days - but always contained
the least amount of ensemble members, generally presenting under
20% of the outcomes. However, latest WPC clusters drops over 30% of
the members into this pattern. Another cluster has a similarly
anomalously strong trough across the western great lakes (which the
GFS prefers). The last 2 clusters are not as strong with the trough,
have more of a positive tilt, and are situated southwest/south of
the local area. That is were the bulk of the EPS members lie. All
told, a lot of variety in the models and a lot of room for different
evolutions as we move into next week.


Outside of the rainfall for Mon night/Tue, confidence low in how
this all plays out. It should be noted that the weak and southerly
location of the trough has been a trend in the EPS over its past few
runs, while the stronger more north/quicker solution has been
favored by the GEFS.

Lastly, depending on how this all plays out, there is the potential
for periods of stronger winds.


&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 939 PM CST Thu Nov 14 2024

MVFR stratus continues across the area late this evening with
this expected to affect LSE and RST until after sunrise Friday.
That said, forecast confidence is low, particularly at RST, as
the western edge of this stratus has been eroding a bit faster
than expected but guidance continues to suggest this rate of
erosion should slow with MVFR ongoing overnight. Have gone with
this solution, but amendments may be needed if ongoing trends
continue.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...None.

&&

$$

DISCUSSION...Rieck
AVIATION...Ferguson