Area Forecast Discussion

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FXUS63 KMQT 172250
AFDMQT

Area Forecast Discussion
National Weather Service Marquette MI
650 PM EDT Fri May 17 2024

.KEY MESSAGES...

-Breezy south winds and very warm temps in the 70s and 80s Saturday 
ahead of a cold front that will bring scattered afternoon/evening 
thunderstorms. The strongest storms may contain small hail. 

-Borderline elevated fire weather expected this weekend with breezy 
conditions on Saturday then very dry with light winds on Sunday.

-More rain chances early to middle of next week as a series of 
disturbances moves across the area in a developing warm and moist 
southerly flow.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 346 PM EDT Fri May 17 2024

Water vapor imagery and RAP analysis show that the shortwave trof 
that brought shra to the area yesterday thru last evening has lifted 
across northern Ontario. In the wake of that wave, westerly flow 
prevails into Upper MI. An area of sct -shra has been moving out 
across western Lake Superior to the Keweenaw in an area of 
waa/isentropic ascent. The -shra that have reached the Keweenaw are 
really not much more than sprinkles. Elsewhere, skies cleared out 
quickly in the morning, but daytime heating led to a blossoming of 
cu. Stabilizing marine layer off of Lake MI and Lake Superior are 
evident, preventing cu. Temps have risen into the low/mid 70s F in 
the interior. Closer to the Lakes, temps range down into the 50s 
with some 40s F at lakeside locations along Lake Superior.  

Aforementioned waa/isentropic ascent will continue to support some 
-shra/sprinkles across the Keweenaw over the next few hrs. MLCAPE 
has increased to 100-200j/kg into far western Upper MI. This limited 
instability may support development of a few shra/tsra late 
aftn/early evening, less than 25pct chc. During the night, vigorous 
shortwave that is now over nw MT will swing out to ND. While the 
main low-level jet in response to this wave will develop ne thru MN 
during the night, there will be a modest increase in 850mb winds 
further ne across western Lake Superior this evening. The resulting 
increase in waa/isentropic ascent should yield some shra/tsra later 
this evening across northern portions of western Lake Superior and 
possibly also nw Upper MI, mainly the Keweenaw where fcst reflects 
30pct chc. Shra/tsra associated with the main 40-50kt low-level jet 
developing across MN would reach western Lake Superior late tonight. 
Expect min temps tonight ranging from the lower 40s on the east side 
of the Keweenaw and near Lake MI to 55-60F in the southerly wind 
downslope areas far w (Ontonagon/Gogebic counties).

&&

.LONG TERM /SATURDAY THROUGH FRIDAY/...
Issued at 523 PM EDT Fri May 17 2024

Highlights of the extended forecast include very warm conditions 
this weekend with the potential for strong thunderstorms and 
elevated fire weather conditions at times. Focus shifts to 
widespread rain potential early/mid next week in addition to a 
transition to below normal temperatures by late next week. 

Beginning Saturday, the UP will be squarely in the warm sector of a 
surface low near Lake Winnipeg with 850 mb temps of 14-16C spreading 
across the UP. This unseasonably warm air mass will support high 
temps in the lower to mid 80s with the highest readings (84F+) for 
downsloping areas along west-central portions of our Lake Superior 
shoreline. Breezy south winds in the tightening gradient ahead of 
the approaching frontal boundary could gust as high as 30-35 mph, 
especially for areas favored by southerly downsloping across Gogebic 
and Ontonagon Counties. Southerly winds off Lake MI keeps the 
eastern UP mostly in the 70s to perhaps even 60s closest to Lake MI. 
ECMWF EFI highlights the unseasonably warm temps well, and like the 
model soundings, also hint at the possibility of unseasonably strong 
winds across the far west. The combination of hot temps and 
gusty winds indicates borderline elevated fire wx conditions 
with the main limiting factor being RH values mainly staying 
above 30%. However, it's worth noting that recent RAP/HRRR guidance
mixed deeper than regional/global models allowing for stronger 
winds and lower RHs below the 30% threshold, but was not 
confident enough in those solutions at this time. 

Timing of the incoming cold front appears increasingly favorable for 
thunderstorm chances across the western UP Saturday evening. The 
latest HREF means show QPF moving into the Ironwood area 20-21Z, 
with hourly rainfall rates peaking about 1-2 hours later (~6 PM). 
Strong frontal forcing should produce a narrow line of storms 
that steadily marches east suggesting peak rainfall rates may 
provide more accurate timing. The peak rates reach the Marquette
area around 3Z (11 PM) then exit the eastern UP around 8Z (4 
AM). MLCAPE values increase into the 500-1000 J/kg range in a 
narrow corridor ahead of the front with 30+ kts of bulk shear 
indicating potential for a few stronger storms. The SPC outlooked
western portions of our area with 5% chances for large hail and
damaging winds accordingly. It's worth noting the 12/18z HRRR 
runs that developed strong outflow wind gusts in the vicinity of
the Porcupine Mountains, but that may be an artifact of the 
deeper mixing mentioned above. 

A much drier air mass and associated surface ridging follows the 
frontal passage for Saturday night into Sunday. Abundant sunshine 
and light winds will result in deep mixing into extremely dry mid-
levels on Sunday. For example, the 12z NAM fcst sounding valid at 
18z Sunday in Ironwood indicated a -40C dew point at 779 mb. 
Fortunately, surface dew points won't mix that low, but still 
incorporated the local mixed dew point tool to cut back on Td values 
resulting in min RHs closer to 20% across the interior portions of 
the cwa. The good news is light winds less than 10 mph should ease 
fire wx concerns, because critically low RHs appear nearly certain 
across interior portions of the UP. Lake breeze activity should 
limit fire wx concerns along the shorelines due to cooler temps in 
the 60s and higher RHs. Expect max temps reaching near 80F over much 
of the interior west half and mid to upper 70s over the interior 
east half. 

Models advertise a mid-level trough amplifying over the western 
CONUS early next week. Periodic shortwaves in a warm/moist sw flow 
developing ahead of the western CONUS trough will bring more rounds 
of showers into the area early to mid next week. The first shortwave 
moves through late Sun night into Monday bringing the initial batch 
of showers for early next week. It looks like the best chance for 
widespread and possibly heavy rainfall will be in the late Tue into 
Wed time frame when models indicate more substantial shortwave 
energy lifting across a nearly stationary frontal boundary draped 
across the area. Model uncertainty increases later into the week but 
it certainly looks possible that some scattered showers could linger 
at least into Thursday as temps trend cooler in the wake of the 
midweek low pressure system/frontal boundary pushing off to the 
east.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/... 
Issued at 649 PM EDT Fri May 17 2024

VFR will prevail thru this fcst period at IWD/CMX/SAW. Pcpn 
associated with a cold front will move into IWD and CMX late in the 
period.  Gusty winds to 20+kt will then set in at IWD and CMX mid to 
late morning on Sat. 

&&

.MARINE...
Issued at 523 PM EDT Fri May 17 2024

Stable conditions leads to light winds of generally 20 knots or less 
continuing into early next week, except for Saturday afternoon and 
evening when southerly gusts of 20-25 kts are forecast ahead of an 
approaching cold front. Thunderstorms are possible this evening over 
western and north-central portions of Lake Superior as a warm front 
lifts across the lake. More thunderstorms are likely on Saturday 
into Saturday night as the same system's cold front moves across the 
lake. These storms may contain small hail, gusty and erratic winds, 
and frequent lightning. More thunder chances return late Sun night 
into Monday and then again late Tuesday into Tuesday night as a 
series of disturbances track northeast across the Upper Great Lakes 
region. Patchy fog is occurring over eastern portions of the 
lake and may become locally dense in areas of rainfall until 
the cold front brings a much drier air mass over the lake
Saturday night.

&&

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

&&

$$

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