Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 181750
AFDMQT

Area Forecast Discussion
National Weather Service Marquette MI
150 PM EDT Tue Apr 18 2017

.SHORT TERM...(Today and tonight)
Issued at 510 AM EDT TUE APR 18 2017

The latest water vapor imagery and RAP analysis shows a well-defined
shortwave moving through ND which models have progged to move thru
the Upper Great Lakes this afternoon and evening. Satellite imagery
shows a line of shra/tsra along the system`s associated cold frontal
boundary from the eastern Dakotas into north central NE. Convection
over the Plains is sustained by lift at nose of 50 knot h85 jet
and MUCAPEs to 1000 j/kg and mid-level lapse rates of 7-7.5 C/KM.

As this shortwave moves east into the Upper Great Lakes this
afternoon deep layer forcing (q-vector convergence) out ahead of
shortwave along with increasing moisture transport and lift at nose
of 50 kt low-level jet will allow numerous shra to spread into the
west half of Upper Mi this morning. Forecast track of shortwave and
associated sfc low across western Upper Mi this afternoon and into
north central Lake Superior this evening will tend to lift best
dynamics/deep moisture northward as the system and associated
frontal boundary move east across Upper Mi. As a result models tend
to show diminishing coverage to showers later into the afternoon and
evening over the east half of Upper Mi. Generally taper pops from
likely to categorical west this morning to lower chance pops east
half this afternoon and evening. South to southeast winds will get
somewhat gusty across area especially for downsloping locations
along Lake Superior where gusts could reach 30 mph. Took out mention
of thunderstorms for far west and along the Wi border as models show
diminishing instability with axis of steep mid-level lapse rates
generally falling apart or sliding south as system moves into the
area. Highs today will be generally in the 50s...except upper 40s
along Lake Mi and the Keweenaw in se flow.

Tonight...The lingering area of showers, mainly over the e half,
associated with the passing shrtwv/sfc low pres and cold front wl
diminish and end by about 06Z as larger scale subsidence/mid lvl
drying follow the exiting disturbance. Min temps will range from
the mid 30s west half to lower 40s east and southeast.

.LONG TERM...(Wednesday through Monday)
Issued at 417 AM EDT TUE APR 18 2017

Focus is on a system that has potential to bring moderate to heavy
snow to portions of the area Wed night through Thu.

Current guidance shifted north substantially from 24 hours ago but
has been more steady for the last 2-3 runs. As it looks now, a
shortwave will drop into the area and deepen over the CWA, causing a
SFC low to move S of the CWA. This will lead to widespread precip,
with the greatest amounts falling as rain out ahead of the SFC low
and also in an area of deformation to the left of the SFC-850mb low
track (this precip is more likely to be snow, so that`s what was
looked at the closest). Strong dynamic cooling and wet-bulb cooling
will quickly turn precip to all snow over mainly the north-central
and west late Wed night into Thu. Upslope flow will add to forcing
and snowfall amounts over the north-central. Several inches of wet,
heavy snow is possible over the north-central since that`s where the
deformation zone is shown to set up and coincides with the upslope.
Impacts would be on the higher end since the wet, heavy snow would
fall moderately to heavily at times during the commute times
(possibly both morning and evening), so advisories and/or warnings
may be needed.

Some thoughts on confidence with the system: There a lot of
variables at play, especially since ptype is borderline between
rain and snow. The track of the system could change
- changing coverage and amounts of snow and even ptype, the strength
could change - leading to lower or higher amounts and differences in
ptype. Plenty of uncertainty still exists given the volatility of
the situation. Stay tuned.

Changes from the blended initialization for this system:

1. Decreased MaxT values using a GFS/NAM blend to lower values into
the mid 30s Thu.
2. Increased QPF some to show a little more upslope enhancement
north-central.
3. Snow ratios were way too low due to previous forecast and
incorrect zero values from the ECMWF and CMC. Increased those using
a NAM/GFS blend.
4. Increase ProbIcePresent using a GFS/NAM blend, only increasing
above SuperBlend. The values were too low due to previous forecast
inclusion (previous forecast was farther south with less ice
probability).
&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 149 PM EDT TUE APR 18 2017

Low pressure lifting northeast across western portions of Upper
Michigan this afternoon and evening will spread rain across all
terminals, with KIWD/KCMX being impacted already this afternoon and
KSAW later this afternoon and evening. The increasing pressure
gradient across the area will work in concert with falling
precipitation to mix down gusty winds through the rest of the
afternoon. Visibilities will also be impacted from the rain as they
may drop down into the MVFR category within the heavier pockets of
rain. The rain will come to an end at KIWD/KCMX later this evening,
and may linger into the overnight hours at KSAW. Behind this system,
expect low stratus to fill in with ceilings possibly dropping down
into the LIFR category at times. Depending on how deep the stratus
can grow, we could see some lingering drizzle in locations that see
remnant deeper moisture. As a 45 to 55 knot low-level jet tracks
northeast across central and eastern portions of the area tonight,
have included mentions of low-level wind shear at KSAW as the
surface winds are expected to relax a bit. With gusty northwest
winds expected to also filter southward across the area tonight into
Wednesday morning, the boundary layer looks like it will be a bit
too mixy for the development of any visbility restrictions at this
time.
&&

.MARINE..

Southeast winds of 10 to 20 knots this morning are expected to
increase to gale force to 35 knots over at least the e half of Lake
Superior this afternoon as the gradient tightens between the
departing high pres and a low pres tracking thru MN toward the
Upper Lakes. Gale warnings are in effect for the impacted areas.
Winds will shift to the nnw up to 25 knots late tonight before
further diminishing on Wed as trailing high pres crosses
Ontario/Lake Superior. Depending on the track of another low pres
approaching the Lower Great Lakes on Thu, ene winds up to 25-30
kts will be possible then. As high pres builds back into the Upper
Lakes late in the week following this low pres, winds will
diminish under 25 kts.
&&

.MQT WATCHES/WARNINGS/ADVISORIES...
Upper Michigan...
None.
Lake Superior...
  Gale Warning until 11 PM EDT this evening for LSZ266-267.

  Gale Warning until 8 PM EDT this evening for LSZ244-245-264-265.

Lake Michigan...
None.
&&

$$

SHORT TERM...Voss
LONG TERM...Titus
AVIATION...Ritzman
MARINE...Voss



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