Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 161736
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1236 PM CDT Wed May 16 2018

Updated aviation portion for 18Z TAF issuance

.SYNOPSIS...
Issued at 355 AM CDT Wed May 16 2018

Warm today, scattered showers across the north tonight, then
cooler Thursday. The next substantial chance for a widespread rain
won`t arrive until the weekend.

A pretty typical late spring pattern is in place across North
America. The main westerlies are across Canada and the far
northern U.S., with a trough over the eastern Pacific, a ridge
near the West Coast, and a deep trough over eastern Canada. A
secondary, much weaker branch of the flow is across the central
and southern CONUS. The most distinct feature within it is a
trough near the West Coast. Changes during the forecast period
will be subtle.

Day-to-day temperature variations will result from the passage of
frontal systems through the area. But after a warm day today
(temperatures 10-16F degrees above normal) readings probably
won`t stray too far from normal. The primary opportunity for
precipitation will occur during the upcoming weekend as a frontal
system crosses the area and interacts with energy ejecting out of
the trough over the Southwest. Amounts are tough to gauge, but
movement of the system appears slow enough to allow for at least a
couple rounds of convection. So the best guess right now is for
near normal amounts for the period.
&&

.SHORT TERM...Today...Tonight...and Thursday
Issued at 355 AM CDT Wed May 16 2018

Surface ridge extending back westward into the area from an
anticyclone over New England was bringing quiet weather early this
morning. That will continue through the day for most of the area.
Warming aloft supports max temps reaching the 80s for all but the
immediate lakeshore.

A frontal system over southern Ontario was generating scattered
showers early this morning. The main forecast concern is the
precipitation potential as the front reaches the area this
evening. Negatives for precipitation will be a lack of large scale
forcing, rising upper heights, and only modest mid-level lapse
rates (6-6.5K/km). Low-level moisture will also be limited as the
north will probably mix out again this afternoon. But a small
area of >50F surface dewpoints had pooled along the boundary, so
moisture may increase a bit as the front arrives. Bumped PoPs up
to mid-range chance (~40%) across the far north during the mid-
evening. Soundings will have somewhat of an inverted-V signature,
so gusty winds are possible with the showers and storms, though
severe gusts are not anticipated.

The showers should wane after sunset as the boundary layer
stabilizes. But the front is likely to accelerate south down the
bay and lakeshore. Increased winds right behind the front and
sharpened the wind shift from what was in the starting grids.

Quiet weather will return Thursday, though gradient northeast flow
will keep temperatures cooler than today. Some of the models
generated precipitation over the western part of the forecast area
during the afternoon, as return flow develops over the boundary.
That seemed a little quick, but left low PoPs in across the far
northwest part of the forecast area since that region will be
most at risk of something moving in from the west, where the
better forcing will be located.

.LONG TERM...Thursday Night Through Tuesday
Issued at 355 AM CDT Wed May 16 2018

Main concern for this period will be the fire weather concerns on
Friday, then the chances of showers and storms over the weekend.
Drier air is then expected to return again early next week with
more low relative humidity readings across the north. On Friday,
another warm day is expected with high temperatures in the 70s
to around 80 with low afternoon relative humidity readings away
from the bay and lake. Wind gusts to around 25 mph are anticipated
based on the mixing heights and winds aloft on the bufkit
soundings.

The next system will bring a small chance of showers to far
north-central Wisconsin late Friday night, but the bulk of the
rain should be on Saturday into Saturday night. Cloud cover, rain
and cooler temperatures should alleviate the fire danger on
Saturday. The best chance of thunderstorms appear to be across
central and east-central Wisconsin. Saturday afternoon into Sunday
morning. At this time, severe storms are not anticipated with
this system. The rain should exit much of the area Sunday morning
with a lingering chance across east-central Wisconsin Sunday
afternoon. The latest ECMWF has trended slower with the departure
of the system, thus may need to raise rain chances across the east
Sunday afternoon in the next few days if this trend continues.

High pressure builds into the region for Monday and Tuesday,
bringing seasonable temperatures along with low afternoon relative
humidity readings. Lowest humidity readings are expected to be
found across the north.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1235 PM CDT Wed May 16 2018

Scattered cu field in the heat of the afternoon across northern WI
will persist for the rest of the day ahead of a cold front that is
dropping south out of Lake Superior. As the front approaches late
this afternoon, there is a chance for showers/isolated
thunderstorms over the north. The chance of precipitation is
forecast to continue into this evening over northern WI as the
front moves farther south through the state. Despite the presence
of the front, VFR conditions are expected to persist. Behind the
frontal boundary, winds are forecast to shift to the northeast and
become gusty across eastern WI in proximity to the bay and Lake
MI late this evening. Thursday appears to be a dry day with the
region on the southwest flank of high pressure situated over
eastern Canada.
&&

.MARINE...
Issued at 355 AM CDT Wed May 16 2018

Although the duration will probably be too brief to warrant a
Small Craft Advisory, a short period of hazardous conditions is
likely on the lake and especially on the bay during the middle to
late evening as the front surges south. Northeast winds could
gust to around 25 kts for a couple hours after FROPA. That
situation can be handled by a Marine Weather Statement once the
exact timing becomes clearer.
&&

.FIRE WEATHER...
Issued at 355 AM CDT Wed May 16 2018

Elevated fire weather conditions will continue across northern
Wisconsin through Friday, especially during the afternoon and
early evening hours due to warm temperatures and low humidity
along with fuel moisture values reaching their typical seasonal
lows. Wind gusts across the north could reach 20 mph today and
around 15 mph on Thursday. The worse conditions are expected on
Friday as some locations could see wind gusts around 25 mph.
The fire weather conditions should improve on Saturday due to
cloud cover and increasing chances for rain. Low relative humidity
readings are expected again early next week.
&&

.GRB WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SYNOPSIS.......Skowronski
SHORT TERM.....Skowronski
LONG TERM......Eckberg
AVIATION.......Kallas
MARINE.........Skowronski
FIRE WEATHER...Eckberg



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