Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Marquette Michigan
806 PM EDT Wednesday Aug 20 2014

Short term...(this evening through thursday)
issued at 344 PM EDT Wednesday Aug 20 2014

An upper low located over eastern Upper Michigan has continued its
slow eastward progression this afternoon...with a broad 1009mb low
stretching from Eastern Lake Superior through northern lower
Michigan. Abundant moisture wrapping around these lows has led to
plenty of low clouds over the entire area. The fog that
occurred over the Keweenaw peninsula this morning has
diminished...but webcams along the Lake Superior shoreline in the
Keweenaw and also at Munising indicate dense fog over the lake.
That matches observations from SHIPS through the day over
central/Eastern Lake Superior through the day. There are still a
few light showers/drizzle over southeastern Marquette County and
northwest Delta County...along with portions of Schoolcraft/Luce
counties. Afternoon temperatures are generally in the 60s across
the u.P. And is about 10 degrees below normal.

The upper trough and associated surface low will drift to the
east-southeast tonight over Lake Huron and then into far southeast
Ontario on Thursday. As this occurs...it will pull the deep
moisture and forcing out with it and leave some lingering low level
moisture over at least the northeast two thirds of the County Warning Area for
tonight. Thus...will opt to stick with persistence with the low
clouds over much of the area and only try to scattered the clouds
out southwest of a line from Ontonagon to Iron Mountain. With the
amount of low level moisture present...expect fog to develop again
tonight near Lake Superior and over the east. Those areas will be
aided by light flow off Lake Superior and some upslope forcing to
aid the low clouds. Think the lowest visibilities would be over the
Keweenaw peninsula where they have the favorable easterly flow
developing overnight...but it will largely depend on the extent of
the fog over south Central Lake Superior (uncertain at this point).
In addition...if some clearing occurs over the far southwest...that
may aid in fog development outside of where the best low level
moisture is located.

The developing broad low over the central/northern plains will be
the item of focus for Thursday...as it slowly lifts northeast
towards the County Warning Area. Latest trends in the 12z models is for the low and
warm front to be a little slower and suppressed to the south than
seen yesterday. That farther south trend will likely keep much of
the precipitation to the south (along the warm front in
northern/central wisconsin) or west (tied with the shortwave lifting
northeast through northern minnesota) of the County Warning Area during the daytime
hours on Thursday. Will slowly spread slights and then chance probability of precipitation
northeast over the southwest third/half of Upper Michigan during the
afternoon hours...largely tied to the broad middle-level warm air advection moving
over that area. Elsewhere...expect a generally mostly cloudy sky due
to the departing low level moisture and increasing middle level
moisture. Will continue to mention some thunder chances during the
afternoon over the southwest third/half...but with the latest trends
in the models keeping the warm front a little farther to the
south...think severe chances are a little lower than thought
yesterday. 0-6km shear is marginal (around 30kts) and with 350-800
j/kg of mu/ml cape nosing into Gogebic County...think that area
would have the best opportunity for stronger storms late Thursday
afternoon if they develop farther to the north than expected.

Long term...(thursday night through wednesday)
issued at 439 PM EDT Wednesday Aug 20 2014

Thursday night...the northern lakes will remain atop and just to the east of a
broad ridge over the central Continental U.S.. ongoing convection into northern WI
north of a warm front from southern Minnesota into central and southern WI is expected
to gradually edge toward and into west and southern Upper Michigan as 850-700
mb Theta-E advection increases. However...model trends and forecast
position of the 850 mb warm front suggest that shortwaves
and associated areas of rain showers/thunderstorms and rain are expected to slide more to the
east-southeast...keeping the best chance for precipitation mainly near the border
compared to earlier forecasts that brought the precipitation and higher quantitative precipitation forecast more
aggressively to the NE. With only marginal effective shear and MUCAPE
values forecast to 800 j/kg the main hazard will be locally heavy rain
as precipitable water values climb to near 2.0 inches.

Friday into Sat...as the middle/upper level trough digs into The
Rockies...the downstream ridge is expected to gradually slide to the
east to the western lakes by late Sat. Although plentiful moisture
remains over the region with potential for some precipitation...any stronger
convection and heavier rain should remain well to the south(srn Minnesota
and western WI or northern il)in closer proximity the surface-850 mb front and
greater instability. Temperatures...in the 70s...will will be held in check by
considerable cloud cover despite 925 temperatures in the 18c to 20c range.

Sun-Mon...models suggest that a stronger shortwave will lift from
the Utah/Wyoming toward southern Manitoba...helping to flatten the ridge and
pushing a front into the western lakes as a surface low moves also moves
into southern Canada. Confidence with details in the developing pattern
are low given significant model differences. The European model (ecmwf)/Gem were
faster in bringing the front through Upper Michigan compared to the
GFS/gfsens.

Tue-Wed...low confidence remains with both temperatures and additional precipitation
chances that will depend on the position of the frontal boundy
that could either move toward the southern lakes(ecmwf/gem) or remain
near the area as another signficant shortwave approaches the area(gfs).
&&

Aviation...(for the 00z tafs through 00z Thursday evening)
issued at 740 PM EDT Wednesday Aug 20 2014

An upper level low spinning near Sault Ste Marie will slowly move
east late tonight into Thursday. Abundant low level moisture rotating
around the low will produce MVFR/IFR conditions this evening lowering
to LIFR overnight at the taf sites and visibilities may even drop to
airfield Landing mins at kcmx in upslope easterly flow.

Expect slow improvement of conditions to MVFR or VFR by early Thursday afternoon
with combination of diurnal heating and low continuing to move east
of the region. Included vcsh at kiwd late in the afternoon with the
possibility of warm air advection rain showers forming ahead of advancing low pressure system
over the plains.

&&

Marine...(for the 4 PM Lake Superior forecast issuance)
issued at 344 PM EDT Wednesday Aug 20 2014

Overall...relatively weak pressure gradients will keep winds below
20 knots through this weekend. The strongest winds will occur over
the western third of Lake Superior on Thursday and Thursday
evening when northeast winds will focus along the western
shoreline. In addition...patchy to areas of fog will occur over
portions of Lake Superior through the upcoming weekend. Ship observation
and webcams indicate dense fog over portions of central/Eastern
Lake Superior and have extended the dense fog advisory into the
evening. This may need to be extended further depending on the
evolution and observations this evening. Additional moisture will
surge north Thursday afternoon and remain through Sunday...producing
additional opportunities for fog over Lake Superior.
&&

MQT watches/warnings/advisories...
Upper Michigan...
none.
Lake Superior...
dense fog advisory until 2 am EDT Thursday for lsz243-244-
249>251-264-266-267.

Lake Michigan...
none.
&&

$$

Short term...srf
long term...jlb
aviation...Voss
marine...srf

National Weather Service Glossary of Abbreviations