Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 210908

Area Forecast Discussion
National Weather Service Marquette MI
508 AM EDT Sun Apr 21 2019

.SHORT TERM...(Today and tonight)
Issued at 507 AM EDT SUN APR 21 2019

...Potential for moderate/heavy rainfall could cause or aggravate
existing flooding tonight into Tuesday for northwest half of U.P...

WV imagery and RAP analysis indicated a mid/upper level ridge from
the southern Plains to the northern Great Lakes between lows over
Hudson Bay and  the Ohio Valley. A shortwave trough sliding through
northern Ontario has pushed a cold front into western Upper Mi early
this morning. Some isolated showers have accompanied the front
into western Upper MI.

Today and Tonight, the cold front will slowly sag through the rest
of Upper Michigan with the stronger 850-700 mb fgen and higher pcpn
chances lingering over the northwest half per consensus of 00Z model
guidance. Stronger fgen response and heavier pcpn signal will occur
late today into Monday over the northwest half of Upper Mi and is
tied to increasing upper divergence in the right entrance region of
120 kt 300-250 mb upper jet over northern Ontario. Along the stalled
out frontal boundary, the baroclinic zone strengthens aloft with
increasing 295-300k isentropic ascent across the boundary, PWATs
around an inch, and warm cloud depths approaching 9kft which will
aid in additional lift/support for a focused area of heavier rain
over the northwest half of Upper Mi. A few of the SREF plumes of
total QPF at Houghton through Mon suggest the possibility of total
rainfall exceeding two inches through the event with much of the
rest of the northwest half solidly in the 1.5 to 2 inch range. Given
better model agreement for a moderate to possibly heavy rainfall
event across the northwest half of Upper Mi, decided to issue a
flood watch for Iron-Marquette counties westward tonight through 18Z
Tue as runoff from the heavier rain will aggravate the already
rising rivers or existing flooding from snowmelt runoff.

Otherwise thickening clouds and developing northeast winds today
will result in a strong north-south temp gradient near or behind the
front with highs in the mid 40s north and mid 60s south. Min temps
tonight will range from the mid to upper 30s northwest half to
generally lower 40s elsewhere.

.LONG TERM...(Monday through Saturday)
Issued at 432 AM EDT SUN APR 21 2019

...Moderate to heavy rain early this week results in Flood Watch for
much of west half of Upper Michigan...

Main focus is on potential for widespread moderate to heavy rain
Monday and Monday night as sfc front and associated sfc low affect
the Upper Great Lakes.

Initially shortwave trough starts day Monday over central Plains
with sfc low over eastern Neb and sfc front extending into northern
WI and south central Upper Michigan. PWATs up to 1 inch will be
advecting northward into the front while upper level divergence
increases in right entrance region of jet streak over northern
Ontario. Shortwaves and intervals of stronger H85-H7 moisture
transport along the boundary should support waves of moderate to
possibly heavy rainfall through the day, mainly north of the sfc
front or generally from western to north central Upper Michigan. By
00z Tue, PWAT around 1.25 inches forecast at GRB ends up around 200-
250 pct of normal, so you see what type of deep moisture this system
will have to work with. Also, with ne low-level winds to north of
approaching low, could see rain amounts become enhanced due to
upslope lifting over higher terrain locations. Majority of higher
res short range models (NAM, WRF-ARW and WRF-NMM and the Canadian
Regional) along with the courser global models (including the
Canadian-NH, GFS/GFS FV3 and ECMWF) have come into good agreement
that swath of heavier rain developing tonight will continue off
and on through Monday and Monday evening, before shifting more
toward central Upper Michigan overnight Monday night as stronger
shortwave trough digging in from scntrl Canada kicks the initial
shortwave trough and sfc low/front on to the east.

Not quite sure how long precip hangs on over the east on Tuesday,
but even the slower idea would exit the precip by early afternoon.
Chilly day expected behind the departing sfc low especially near
Lake Superior and east where temps will stay in the low 40s with a
blustery n-nw wind. NAM looks a bit bullish compared to other
guidance on cold air advection Monday night into Tuesday morning
over the west as it would have enough cold air to swich rain to snow
late Monday night. In any case, that NAM idea shows how it cools off
on Tuesday, which should slow the runoff some. Certainly by Tuesday
night as temps drop back below freezing most areas, the runoff
should slow down considerably.

Based on recent reports from COOP and CoCoRAHS observers and NOHRSC
data, snow depths are still around a foot if not locally higher in
the higher terrain of far nw Upper Michigan including the Keweenaw
so any rain with above freezing temps will cause issues. Snow depths
tail off to 5-10 inches over rest of nw Upper Michigan into ncntrl
Upper Michigan, including recent observation of 6 inches here at NWS
Marquette. SWEs in the snowpack remain high with just under 4 inches
observed at NWS Marquette yesterday. Some locations likely still
have 6-8 inches of SWE over the Keweenaw. Given the lingering snow
pack, higher SWE in the snowpack and expected rainfall over the next
48 hours, coordinated with short term forecaster and WFO DLH and GRB
and we put out a flood watch for Iron to Marquette counties westward
as that is where there is good signal and trends in the models of
seeing at least 1 to 1.25 inches of rain by Tuesday morning. Based
on SREF plumes from IWD to CMX to P59, there is certainly potential
for more as those plumes show increasing amount of members exceeding
the mean with many more members now pointing toward totals over 1.75
inches if not even 2.25 inches. With the extent of moisture pushing
into this rather stationary boundary and persistent lifting
mechanisms at work, these higher amounts seem reasonable, at least
in localized swaths.

Should note that the watch was issued for the overall larger scale
areal flooding impacts, not the impacts tied to specific rivers.
Continue to monitor for details of flooding
on specific rivers. Currently we have a handfull of rivers that have
either advisories or warnings in place and it is very likely that
number will expand and ongoing flooding will worsen as we progress
through this week. It is possible some rivers will end up cresting
at levels higher than we have seen for many years.

As for the rest of the extended. Towards the end of the week, a
quick moving shortwave will push a weak cold front across the Upper
Great Lakes, bringing another chance for rain late Wednesday night
through Thursday morning. Temps ahead of the front probably crack
60F over west half on Wednesday and could still be in the 50s most
areas on Thursday. Though mostly temps at night mid to late week
will be below freezing, Wed night temps likely stay well above
freezing in the southerly flow ahead of approaching front.

Another chance for more widespread rain arrives next weekend ahead
of a sfc-H85 low and warm front. Along with the rain, looks like
cool conditions with prevailing east-southeast low-level flow.

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 414 AM EDT SUN APR 21 2019

VFR conditions will continue through tonight. Some showers will
begin moving in at IWD and CMX this morning, but MVFR conditions
at IWD and CMX should hold off until late this morning and at SAW
by late afternoon. Conditions this evening will deteriorate
further at all sites as steady rain moves in and fog develops.

.MARINE...(For the 4 AM Lake Superior forecast issuance)
Issued at 507 AM EDT SUN APR 21 2019

Northeast winds of 20 to 25 knots are expected on the western end of
Lake Superior today into Monday with north winds 20 to 25 knots into
central Lake Superior Monday night into Tuesday. Areas of fog could
develop beginning tonight and continuing into Tuesday as rain and
higher dewpoints are expected to move over the lake.  Otherwise, a
quiet stretch of weather will continue from midweek on as winds
remain below at or below 20 knots.

Upper Michigan...
  Flood Watch from this evening through Tuesday afternoon for

Lake Superior...
Lake Michigan...


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