Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Marquette Michigan
633 am EST Sat Dec 27 2014
Short term...(today and tonight)
issued at 535 am EST Sat Dec 27 2014
Latest water vapor loop and RUC analysis shows a vigorous and
well-defined shortwave lifting NE into southern Minnesota and northwest WI with
developing baroclinic Leaf structure/deformation zone. Middle-level fgen
associated with shortwave stretching from southern Minnesota and northwest WI into the Upper
Michigan helping support light to moderate snow.
Today...mid-lvl fgen stretching across the upper Midwest to the
upper Great Lakes will continue to support moderate snow into early
afternoon. Broad scale synoptic ascent will be aided by upper
divergence in the left exit region of the 300 mb jet. Negative epv
noted on cross-sections within a region of middle-level frontogenesis
could lead to some locally heavier snow bands as surface low tracks just
southeast of area this morning and then across The Straits early this afternoon.
Model consensus quantitative precipitation forecast argues for area of heavier snow just northwest of low
track along and east of a line from Iron River through Ishpeming
where models show strong 285-290k isentropic ascent over a six-hour
period with 4 g/kg mixr advecting across area. This swath of heavier
snow will taper off by middle afternoon as middle-level fgen weakens and
begins to move NE with departing surface low. Model average quantitative precipitation forecast from .25 inch
west to near .5 inch over portions of the central County Warning Area with expected
snow/water ratios from 15/1 west to near 12/1 east half should yield
total snow accumulation from 2-5 inches west to 3-6 inches central.
Since 6 inch amounts should be fairly localized will continue with
Winter Weather Advisory headlines at this point and not upgrade to
warnings...but situation will definitely warrant monitoring this
morning. Also have decided to include Ontonagon and southern Houghton in
advisories as they should be on western edge of heavier quantitative precipitation forecast this morning.
Tonight...with system snow over by late afternoon for east...expect
transition to pure lake effect snow tonight as 850mb temperatures fall from
-10 to -12c in the evening to -12 to -16c by 12z sun under low-level
west to west-northwest flow. Large scale deep layer q-vector divergence will
dominate this evening and NAM/GFS forecast soundings showing inversions
based near 4kft...any developing les will be on the light side over
the Keweenaw. However...later tonight there is a potential for some
weak enhancement as a shortwave passes across northern Ontario and sends
a surface trough S across the upper lakes. GFS/NAM forecast soundings show an
increase in moisture depth and lifting of the inversion. This
enhancement combined with favorable placement of dgz within
convective boundary layer and sharpening of the wind
shift/convergence as trough moves through area could yield moderate
snow late tonight with a quick 1 to locally 3 inches of snow in northern
Ontonagon/Houghton counties where sharpest convergence is indicated
and where upsloping will further aid snow accumulations.
Went on colder side of guidance for min temperatures especially over the
western interior as precipitation water fall to less than .25 inch under light
winds...a period of clearing skies and fresh snow cover. Could see
the typical cold spots near the WI border fall near zero.
Long term...(sunday through friday)
issued at 458 am EST Sat Dec 27 2014
Will start out 12z Sunday with a cold front draped across the far West
County warning area. This will be the focus for light to potentially moderate snow
showers from around Ontonagon up through the Keweenaw peninsula. As
noted in the previous discussion...a les advisory by be needed for
Ontonagon and Houghton counties. Currently forecasting 3-5in of new
snow in this area overnight through Sunday evening. Will hold off on
issuing anything as Ontonagon and S Houghton have been included in
the initial Winter Weather Advisory for today.
With cooler air sliding in...look for snow ratios to be on the
increase...ranging from around 20-23:1 through much of this period.
The surface trough will slowly exit as strong and cold high pressure
pushes from far northwest Canada Sunday to British Columbia/Alberta on
Monday...with a sizable ridge extending into the upper Great Lakes.
The lowering inversion heights /around 850mb at iwd/ will start to
become a concern for moderate/heavy snowfall development from 00z
Monday on across the County Warning Area. Still expecting 12hr snow totals of 1-3in
where the best convergence develops with plenty of moisture still
stuck in our steady northwest flow into Tuesday.
The coldest air looks to swing in Tuesday night...on the order of
-21 to -24c. If winds become more westerly and the interior west half
clears out...this could be a very chilly night. Currently calling
for -5 to -10f lows by daybreak Wednesday under a partly cloudy sky.
There may be a brief reprise from the steady northwest to west-northwest winds
Wednesday into Wednesday night. Probability of precipitation/weather may need to be diminished
from the current forecast for Wednesday night if this trend holds up.
There is plenty of discrepancies between the available guidance at
this point...varying from -20c 850mb air overhead on gusty northwest winds
/up to 40kts at 850mb/ from the 27/00z GFS...and more west-west-southwest winds
with -12c 850mb temperatures off the 26/12z European model (ecmwf). Will keep going
with a general blend between the consensus and our previous forecast
beyond this point given the moderate to lower confidence.
Aviation...(for the 12z tafs through 12z Sunday morning)
issued at 626 am EST Sat Dec 27 2014
Expect IFR/LIFR conditions through early afternoon as low pressure
over southern WI lifts northeast to northern lower Michigan this afternoon.
The low pressure system will bring light to moderate snowfall across
west-central Upper Michigan through early this afternoon. Have shown
lowering trend with ceilings/visibilities as this snow moves
through...with the worst conditions at kiwd and especially
ksaw...where conditions will fall to or remain LIFR into early afternoon.
The low and heavier snow will quickly depart NE as conditions
improve to MVFR by middle afternoon and to VFR Sat evening at kiwd/ksaw.
Colder air moving over Lake Superior tonight in a west-northwest flow will help
develop lake effect snow at kcmx and kiwd late Sat night. Convergent
westerly flow and gustier winds developing could lead to IFR
conditions at kcmx in snow/blowing snow. &&
Marine...(for the 4 am Lake Superior forecast issuance)
issued at 535 am EST Sat Dec 27 2014
Expect generally light winds into this morning as a weak pressure
gradient is in place due to a surface trough stalled over the
Mackinac straits. But as a low lifts northeast of The Straits this
afternoon into Quebec tonight...northwest winds will be on the
increase this afternoon and evening with gusts up to 30kts over east
half. Behind the low...a weak surface trough will remain in place on
Sunday and keep west-northwest winds up to 25kts in place. A high
pressure ridge building into the area on Monday from a strong high
in the northern/Central Plains will lead to northwest to northerly
winds up to 25-30kts through Tuesday. An approaching trough from a
low over Hudson Bay on Tuesday night will lead to a tighter pressure
gradient and westerly gales to 35-40kts late Tuesday night into
Winter Weather Advisory until 7 PM EST this evening for miz006-
Winter Weather Advisory until 4 PM EST /3 PM CST/ this afternoon
Winter Weather Advisory until 1 PM EST /noon CST/ this afternoon