Forecast Discussions mentioning any of "HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 10/22/23

Area Forecast Discussion

National Weather Service Marquette MI

 626 PM EDT Sat Oct 21 2023

.SHORT TERM...(Through Tonight)
Issued at 317 PM EDT SAT OCT 21 2023

Key Messages:
-Rain showers along the WI border wrapping up this afternoon,
followed by lingering lake effect this evening through tonight.
-Breezy conditions in the eastern UP this afternoon before winds
fall back this evening

Water vapor and RAP analysis show a fairly well-defined shortwave
dropping through western Lake Superior and the western UP this
afternoon amid broad NW flow ahead of a the ridge building across
the Intermountain West. Meanwhile, closer to the surface, low
pressure continues to move southeast into the Ohio Valley region,
keeping winds across the UP out of the north. Winds are picking up
across the eastern UP with the resulting tightening pressure
gradient; gusts to around 20 mph should be more common there before
we head into the evening and we decouple.

Showers this afternoon continue to move mainly along and south of
the WI border, with just some stray weak radar returns across the
western and central UP. However, with cool northerly flow off of
Superior and 850mb temperatures already at around -4C expected to
continue dropping through tonight, expect some lake effect rain
showers to develop heading into the evening in the north wind- and
upslope-favored areas. Heavy accumulations are not looking very
likely as very dry midlevel air works in. With lake clouds
blanketing most of the area tonight, most of the area should bottom
out in the upper 30s to lower 40s. However, where breaks in the
clouds can occur (with the best potential in the south-central UP),
temperatures closer to the freezing point will be possible.

.LONG TERM...(Sunday through Saturday)
Issued at 434 PM EDT SAT OCT 21 2023

Key Messages:
-Temps not straying too far from normal next week, warmest day
probably on Tue.
-Colder air arriving next weekend thru the first half of the week of
Oct 29 will bring the first bout of LES of the season.
-Periodic pcpn next week, generally focused Tue and late week,
including thunderstorms on Tue.

Medium range models are in good agreement showing a strengthening
mid-level positive height anomaly over the Bering Sea today shifting
e to AK for the middle and end of the upcoming week. Downstream flow
across the U.S. will adjust to this feature, but details of that
adjustment are complicated by undercutting of the anomaly thru at
least the midweek period. However, a general w to sw mid-level flow
will be favored into the Upper Great Lakes next week. Thereafter,
guidance does suggest consolidation of ridging from AK southward
vcnty of the W Coast, which will in turn force trof amplification
downstream thru central into eastern N America next weekend thru the
first half of the week of Oct 29. For Upper MI, this evolution of
the flow will result in temps generally not straying too far from
normal next week. Warmest day should be on Tue as shortwave
approaches. There may be another warmer day late week, depending on
track of low pres lifting out of the western U.S. A definite trend
to colder weather will then occur next weekend into the first half
of the following week due to aforementioned trof amplification. As
for pcpn, expect periodic episodes of pcpn during the upcoming week,
but probably focused on Tue and late week. Although the first
snowflakes of the season have already been observed in Upper MI, at
Herman on Oct 7th, majority of locations will see the first
snowflakes of the season by the end of the month, mostly from LES,
as the expected eastern trof amplification occurs. Looking farther
ahead, recent CFSv2 ensemble means have trended toward more
aggressive warming spreading e into the Great Lakes region during
early and mid Nov. With this only a recent trend, it is for now just
a signal to monitor for persistence, but it is noted that the end of
the GEPS/EPS runs do support the start of warming suggested by the
CFSv2.

Beginning Sun, surface high pres ridge axis will shift across Upper
MI. Always tough at this time of year to get lower clouds to clear
out, and since lower level winds don`t decisively shift sw across
the entire area, stratocu will likely linger for many locations.
Best chc of clearing out of low clouds will probably be over the s
central and maybe w as winds shift light w to sw. Even so, fcst
soundings suggest high clouds spreading over the area, generated
from strengthening waa to the w. Expect highs in the mid 40s to lwr
50s F.

WAA/isentropic ascent will increase Sun night, particularly late,
resulting from a shortwave moving across Manitoba into northern
Ontario and a second wave moving from the central Plains to the
Upper Mississippi Valley. This ascent should generate some sct shra
which may approach w to s central Upper MI toward 12z Mon. With sfc
high pres ridge yet to exit the e Sun evening, there will be the
opportunity for temps to fall quickly if low clouds have cleared out
and higher clouds haven`t increased or thickened too much. Temps may
slip into the mid/upper 20s F eastern interior. Min temps will range
up to around 40F far w where return flow will be increasing along
with clouds.

On Mon, the Upper Mississippi Valley wave will pass just s of Upper
MI while the other passes well to the n. With the continued
waa/isentropic ascent regime across the area, sct shra can`t be
ruled out. NAM is most aggressive in showing shra spreading across
much of Upper MI, aided by weak cape for parcels lifted from within
the 800-700mb layer. Given the weaker signal from other models, pcpn
mention will be kept mostly in the low chc range for now, but
highest s central. High temps should range mostly from 50-55F.

During Mon night/Tue, a minor shortwave or two will lift from the
central Rockies to northern Ontario. Responding low-level jet of 30-
40kt into the Great Lakes region will spread anomalous warmth at
850mb into the area (850mb temps rise to around 12C). Precipitable
water also increases up to around 250pct of normal. With the
anomalous warmth/moisture and steep mid-level lapse rates of 7.5-
8C/km, anomalous instability also develops with potential cape of
1000-2000j/kg avbl for parcels lifted from near or above top of sfc
based stable layer. Best chc of shra/tsra will be during the day on
Tue into Tue evening as weak sfc low pres wave lifts across the
area. Given the potential cape, there could be a low risk of hail
depending on what the effective deep layer shear is for the elevated
storms. Heavy downpours certainly possible with the high
precipitable water values. Expect highs in the mid 50s to lwr 60s F
on Tue.

Passage of the weak sfc low pres wave pushes the anomalous moisture
s and e Tue night into Wed, but with track/timing of waves across
the U.S. uncertain due to some degree of the flow undercutting the
AK positive height anomaly, fcst becomes much more uncertain after
Tue. From midweek onward, the operational GFS and its ensembles are
not favored given the better agreement btwn the GEPS/EPS. Trend
should be toward drier weather at least on Wed in the immediate wake
of the Tue/Tue night wave. Ensembles are suggestive of a low pres
wave lifting ne across the Upper Great Lakes at some point Thu/Fri,
bringing the next better chc of shra to the area. For now, ensembles
are not supportive of a wrapped up/deep low moving across the area.
Following this wave, colder air will begin to push se into the area,
especially later weekend when one last wave may lift ne thru the
Great Lakes. The colder trend will set the stage for increasing nw
flow lake effect showers Fri thru the weekend with snow pytpe
becoming more predominant with time.
&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 626 PM EDT SAT OCT 21 2023

MVFR conditions will persist for most of the period at SAW and IWD,
with VFR conditions moving into SAW Sun afternoon. CMX will be MVFR
into the evening before going to low VFR through the rest of the
period.
&&

.MARINE...(For the 4 PM Lake Superior forecast issuance)
Issued at 434 PM EDT SAT OCT 21 2023

NNE wind gusts to 20-25kt are possible this evening across eastern
Lake Superior. Otherwise, high pres ridge drifting across Lake
Superior late tonight thru Sun evening will be accompanied by a
period of winds under 15kt. During Sun night, s winds on the
backside of the ridge will increase, reaching up to 20kt by daybreak
Mon. A further increase to up to 25kt will occur on Mon, mainly
across the e half of Lake Superior. Winds will then settle back
during Mon night as pres gradient weakens. A low pres trof will then
move se across the lake during Tue and Tue night. Winds will be
under 20kt during this period. Winds should remain mostly under 20kt
on Wed as well since the exiting low pres trof will remain on the
weak side. A low pres wave should then lift thru the Upper Great
Lakes at some Thu/Fri. While a period of stronger winds will occur,
gale potential appears very unlikely at this point.
&&

.MQT WATCHES/WARNINGS/ADVISORIES...
Upper Michigan...
None.
Lake Superior...
None.
Lake Michigan...
None.
&&

$$

SHORT TERM...LC
LONG TERM...Rolfson
AVIATION...07
MARINE...Rolfson