Area Forecast Discussion
Issued by NWS Marquette, MI
FXUS63 KMQT 212352
Area Forecast Discussion
National Weather Service Marquette MI
752 PM EDT SUN AUG 21 2016
.SHORT TERM...(This evening through Monday)
Issued at 403 PM EDT SUN AUG 21 2016
12Z raobs/latest wv imagery and RAP analysis show an upr trof axis
extending fm Hudson Bay into the Great Lks downwind of upr rdging
over the Rockies. A diminishing llvl NW flow btwn departing sfc lo
pres in Quebec and an aprchg hi pres rdg stretching fm the Scentral
Plains into MN is beginning to advect drier llvl air shown on the
12Z INL raob into the Upr Lks. Sfc dewpts have fallen as lo as 39 at
Houghton at 19Z. However, the interaction btwn a weak shrtwv aprchg
fm the NW and lingering llvl mstr shown on the 12Z GRB raob under
h85 thermal troffing is still resulting in a good deal of
instability cu/sc and some showers ahead of the line of sharper
drying. Looking farther upstream, there is an area of mid clds over
Manitoba spreading toward NW MN in the SW flow/WAA on the backside
of the sfc hi pres rdg.
Main fcst concerns in the short term focus on cld trends and temps.
Late Today/Tngt...The combination of arriving drier llvl air/incrsg
acyc flow/loss of daytime heating wl cause lingering clds/showers to
give way to clrg skies W-E by sunset. Although skies wl then be
moclr with the passage of sfc rdg axis until some mid clds arrive
over the NW late associated with isentropic ascent depicted on the
305K isentropic sfc, a light NW backing WSW wind under the pres
gradient on the nrn flank of sfc hi pres center passing well to the
S in the OH River valley wl limit the diurnal temp fall a bit and
prevent temps fm reaching current dewpts. Expect the lowest min
temps arnd 45 over the interior Scentral where the gradient wl be a
bit slacker and away fm lk moderation.
Mon...Band of WAA mid clds in the return SW flow behind the
departing sfc hi pres rdg and associated with sharper isentropic
ascent/moistening on the 305K sfc are fcst to impact mainly the N
half of the cwa. Most of the numerical models also generate some
pcpn, but these showers are fcst to remain mainly over Lk Sup and
away fm drier near sfc air over the land cwa and farther S closer to
the sfc hi center in the OH Valley. With incrsg pres gradient, SW
h925 winds are fcst to reach aoa 25 kts during the aftn over the W.
In the presence of daytime heating/ mixing, expect some gusty winds,
especially over the W. H85 temps are fcst to reach near 16C over the
NW half in the aftn, supporting hi temps in the lo 80s in that area.
Expect mid 70s downwind of Lk MI.
.LONG TERM...(Monday night through Sunday)
Issued at 417 PM EDT SUN AUG 21 2016
Deep trof over the Great Lakes today will give way to lower
amplitude flow/weaker height anomalies across the CONUS this week.
There is really only one significant feature of interest for the
fcst area this week, and that is the vigorous shortwave trof now
over British Columbia. This wave will be strongest over the next
couple of days as it tracks basically along the U.S./Canada border.
Once e of the Rockies, medium guidance shows this wave weakening as
it moves into increasingly confluent flow downstream, resulting from
a building ridge from the Gulf Coast States up the E Coast and a
trof lingering in the vcnty of Hudson Bay. The shortwave trof will
reach the Great Lakes on Thu. After the current refreshing change of
air mass, the approaching trof will bring a strong warm up that will
peak on Tue with temps well above normal for late Aug. Warmth will
linger Wed though more clouds/possible pcpn/arriving cold front will
temper highs compared to Tue. Cold front will pass Wed/Wed night,
dropping temps back to normal or perhaps a little blo normal by Fri.
Some warming will then get underway over the weekend. As for pcpn,
there may be some isold/sct convection Mon night due to low-level
jet/strong waa, but the best chc of pcpn will be Wed/Wed night as
cold front approaches/passes in association with aforementioned
shortwave trof. There are indications that another shortwave may
move out of the Rockies and reach the Upper Lakes over the weekend,
bringing a chance of pcpn at that time.
Beginning Mon night, 40-45kt low-level jet moves over the fcst area.
While there is weak theta-e advection over Upper MI, it is much
stronger to the n and ne, suggesting the better potential of
convection will be n and ne of here. Nonetheless, given the fcst
850mb convergence and some instability as well as a majority of
models indicating pcpn, will include schc/low chc pops across
mainly the central and eastern fcst area Mon night.
On Tue, 850mb temps rise to the 17-20C range, and fcst soundings
show air mass becoming capped, preventing development of any sfc
based convection. Have thus maintained a dry fcst for Tue. Based on
mixing heights on NAM/GFS fcst soundings, temps should rise well
into the 80s with some locations probably topping 90F where s to sw
winds downslope. Dwpts rising back to aoa 60F will once again add to
the uncomfort level.
Shortwave trof now over British Columbia will reach ND/Manitoba Wed
aftn. Canadian/ECMWF/GFS indicate a possible lead shortwave lifting
out of the Plains into the Upper Great Lakes ahead of the main trof.
Timing/track/intensity of this feature will certainly be influenced
by convection that it generates, leading to low confidence in pcpn
fcst here on Wed. However, it does offer the potential for a weak
low pres wave or area of enhanced convection to lift into the Upper
Lakes along the cold front moving into the area. Given the
uncertainties, a consensus of recent model runs will be used to
construct pcpn fcst, resulting in chc pops for Wed/Wed night. Looks
a secondary trof/cold front may drop thru the area on Thu, possibly
generating isold -shra.
Sfc high pres following for Fri will provide dry weather. As would
be expected at this time range, timing of next potential shortwave
over the weekend is very uncertain. Right now, it appears dry
weather may linger through Sat aftn.
.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 751 PM EDT SUN AUG 21 2016
Arrival of drier air will result in VFR conditions at all 3 TAF
sites this forecast period. Expect gusty SW winds to develop Mon
afternoon, especially at KIWD as pressure gradient sharpens again
on the northwest flank of the departing ridge.
.MARINE...(For the 4 PM Lake Superior forecast issuance)
Issued at 403 PM EDT SUN AUG 21 2016
As a hi pres ridge axis passes Lake Superior tonight, expect NW
winds up to 20-25 kts this evening to diminish and back to the SW by
Mon morning. The pres gradient is fcst to sharpen on Mon thru Tue
between the departing hi pres and an approaching cold front, so
expect the SSW winds to pick up to 20-25 kts again as soon as Mon
night and continue at times into Wed. Winds will shift to the W-NW
up to 25 kts following a cold frontal passage late on Wed and
continue thru Thu. After the trailing hi pres shifts to the E on
Fri, winds will shift back to the SW.
Beach Hazards Statement until 10 PM EDT this evening for MIZ005-