Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Marquette Michigan
739 am EDT Sat Aug 23 2014
Short term...(today and tonight)
issued at 446 am EDT Sat Aug 23 2014
A fairly amplified upper level pattern over the Continental U.S. For this time
of the year...as upper troughs are located over the northern
rockies and just to the east of the New England states.
Meanwhile...an upper ridge is located in the central Continental U.S....with
the axis running from Missouri north to Western Lake Superior. The
upper ridge near the County Warning Area and the lifting trough over the northern
rockies will be the main influence on the weather for the next
several days. Currently...still seeing scattered/numerous rain
showers over the central part of Upper Michigan to the east of the
upper ridge and just to the east of a weak surface trough
stretching north across central Wisconsin and into western Upper
Michigan. This past evening there were localized areas of heavy
rain in northeast Iron County (radar estimates of 2.5-4.5 inches
around 5-7mi to the northeast of amasa). This area of rain has
weakened slightly and shifted farther to the east (now covering
southern Marquette...central Delta...and far southwest Schoolcraft
counties)...as it appears a weak mesoscale low developed in northern Iron
and southern Baraga counties based off the recent radar loop.
Getting a handle on the evolution of this rain is the main
difficulty for the next several hours of the forecast.
Very subtle features controlling this area of showers...which makes
forecasting it over the next 6hrs difficult. Think the showers are
aided by weak upper divergence in the right rear of an upper
jet...weak 850-700mb q-vector divergence...and convergence along
the weak inverted surface to 925mb trough over west-central Upper
Michigan. Current thought for the rain over the central County Warning Area is for
it to slowly move east and maybe a little north through the morning
hours due to the light flow within the cloud layer. Then...as the
upper ridge amplifies and pushes north towards the afternoon hours
would expect the showers to diminish and shift northeast as low
level southeasterly flow begins to increase. Thus...have followed
that idea for probability of precipitation today although confidence quickly decreases
through the day with the small scale influences on the showers.
Brief heavy rain is still possible (pwat values around 175 percent
of normal...slow movement...and radar instantaneous rain rate
showing 0.75-1.5 in/hr)...but Don/T expect any heavy rains like
was seen last evening. Also for this morning...have seen fog
develop in some locations...especially away from the rain and the
weak mixing it has provided...and that should continue into the
first part of the morning hours before diurnal heating diminishes
Otherwise...expect cloudy conditions for much of the day...as low
level moisture will stay in place for much of Upper Michigan. With
the southeasterly flow increasing through the afternoon...it will
try to focus the deeper moisture to the western third of the County Warning Area.
Some drying in the middle levels could lead to breaks in the clouds
over the eastern third of the County Warning Area. Did leave some slight chance of
showers/thunderstorms over the interior west heading into the
afternoon along the surface trough. Highs will be in the 70s for
most locations and it will be humid with dewpoints in the low-middle
For tonight...expect most of the precipitation to stay off to the west
(due to the upper ridge building over the cwa) and closer to the
shortwave and associated surface low lifting north through the
eastern Dakotas. Kept the probability of precipitation over the western third of Lake
Superior through the night...but did sneak them eastward into
western Gogebic/Ontonagon counties towards sunrise. The increasing
southeasterly flow aloft will likely lead to near surface winds
increasing overnight and help limit the visibility reduction from
fog. Still expect some fog...but think it will trend more towards a
low stratus. Have patchy fog in the favored locations with the
southeasterly wind and areas of fog where the upslope flow will
provide additional lift and put bases near the surface.
Long term...(sunday through friday)
issued at 525 am EDT Sat Aug 23 2014
Trough is currently settling into The Rockies while downstream ridge
is building over the Mississippi River valley. Main piece of energy
in the trough will lift NE into the Dakotas sun and then into northern
Ontario Monday/Monday night. This will suppress and force the ridge
eastward so that the axis extends through the lower lakes to New
England by late Monday. Additional energy will drop into the western trough
early next week...and then the trough will become more progressive for
the remainder of the week. End result of these changes will be
warm/humid weather into the early part of next week...and then a
change to cooler weather for the middle and end of the week as the
trough passes. As for precipitation...there will be some increase in precipitation
potential late sun into Monday as the main piece of energy tracks from
the Dakotas into northern Ontario. However...shortwave will be taking a
track far enough west and north of the area to probably preclude a
significant precipitation chance as height falls will be weak as cold/occluded
front swings east across the upper lakes. Another shot of precipitation may
occur around midweek as western trough progresses eastward.
Beginning sun...during the morning...isolated rain showers/thunderstorms and rain may affect
far western Upper Michigan which will be on the eastern fringe of low-level jet
aimed north through Minnesota. Tail of shortwave lifting through Minnesota into northern Ontario
may provide some support as well. Otherwise...main piece of energy
in the western trough will lift through the western Dakotas toward southern Manitoba.
Surface low will take a similar track...and during the day sun...the
associated cold/occluded front will remain well west of Upper Michigan. By
00z Monday...the roughly north-S oriented front will only be halfway across
Minnesota. With front well to the west...little/no height falls and probably a
capped environment through the afternoon...a dry day should be the rule.
However...opted to include slight chance probability of precipitation far west late in the afternoon as
weak height falls/deep layer forcing begin to spread toward Upper
Michigan. Forecast soundings suggest maximum temperatures generally in the low/middle 80s.
It will be uncomfortably humid.
On Sun night...weak height falls on the order of 20-40m spread into
Upper Michigan as system lifts into southern Manitoba. Frontal boundary will
also push into western Upper Michigan late in the night. With best deep layer
forcing well off to the northwest...believe rain showers/thunderstorms and rain along boundary should
weaken and become less widespread as they accompany front into Upper
Michigan Sun night. Normal nighttime decrease in instability will also
work to diminish precipitation unless convection becomes elevated...though
that is not likely. That said...there will still be sufficient
instability in the presence of 35-40kt deep layer shear to support
the potential of organized strong storms and perhaps a severe storm if
surface based convection holds together.
Frontal boundary continues eastward on Monday...but there is
disagreement on how far east front will move. The GFS holds up front
over the forecast area Monday while the Gem/European model (ecmwf) drift front east...exiting
the eastern forecast area Monday evening. Despite front hanging up...GFS still
shows rather aggressive midlevel drying pushing across the forecast area
Monday. So...consensus supports a trend to dry weather from west to east Monday
into Monday evening. The eastern forecast area will have a chance to build
instability before front and/or middle level drying arrives...so will
show an increase in probability of precipitation over the east for the afternoon while the west becomes
mostly sunny. Deep layer shear of 30-40kt will support potential for
organized strong storms with the potential for isolated severe.
On Monday night...the GFS brings shortwave energy into the upper lakes
in west-southwest flow...and the front it hangs up over the area Monday becomes a
focusing point for new precipitation development. Meanwhile...the Gem/European model (ecmwf)
keep the area dry with frontal boundary farther to the S and east. Will
lean toward the drier scenario for now...with only schc probability of precipitation across
the S and east.
The uncertainty that increases Monday night continues to grow through
midweek. The GFS...as it has for a number of runs...maintains a more
amplified trough progressing across the Continental U.S. With the result being a
more well-defined surface wave in most runs tracking along frontal
boundary into the Great Lakes. The European model (ecmwf) and Gem have been flatter
with the pattern except for a run or two. The flatter pattern
results in the frontal boundary farther S and east with weaker surface
waves tracking along it and thus less precipitation potential for Upper
Michigan...particularly across the north and west. Will continue to utilize a
simple consensus until better model agreement develops. This results
in chance probability of precipitation spreading into the SW forecast area Tuesday and then across the
area Tuesday night through Wednesday night. The late week period should turn dry
in the wake of the trough.
Aviation...(for the 12z tafs through 12z Sunday morning)
issued at 737 am EDT Sat Aug 23 2014
Expect the period of low clouds to continue through the first half
of the daytime hours today...with all three sites at IFR or lower
conditions. But diurnal heating and gradually increasing
southeasterly flow bringing in drier air should help lift ceilings
to MVFR (and potentially VFR at kcmx/ksaw this afternoon) along with
ending the fog. Expect the showers that are currently over central
Upper Michigan to diminish through the day...although kiwd could see
a shower later this afternoon but confidence is too low to include.
Tonight...southeasterly flow will be on the increase and attempt to
limit fog development. Thus...have trended towards lower stratus but
kept visibilities in the MVFR range.
Marine...(for the 4 am Lake Superior forecast issuance)
issued at 446 am EDT Sat Aug 23 2014
A low pressure system lifting north from the Central Plains today
and into southern Manitoba by Sunday morning will lead to a
transition from light northeast winds to east-southeast winds under
20kts by Sunday. As this low moves to James Bay by Monday
night...expect a trough to move east through Lake Superior on Monday
and shifting the winds to a westerly direction by Monday night.
There could be brief periods of gusts to 25kts just ahead and behind
the front. The westerly winds under 20kts will continue through the
middle of next week as the trough slowly sags southeast across the
Great Lakes region.
Moist air over the upper Great Lakes region continues to produce
fog over portions of Lake Superior. As southerly flow develops on
Sunday...more moisture will move over the lake and likely lead to
more widespread fog over the northeast half of Lake Superior. Observation
this past evening indicated some areas of dense fog over the Central
Lake and may need to issue a dense fog advisory if coverage expands.
Drier air arriving behind the front on Monday should bring an end to
the fog on Monday night.