Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Caribou ME
645 am EDT Friday Jul 31 2015
an upper disturbance will cross northern Maine today followed by
a weak ridge of high pressure tonight. Unsettled conditions will
persist Saturday and Sunday with scattered showers and thunderstorms.
Near term /through tonight/...
630 am update...the fog is starting to burn off and lift west/the
help of the sun. Temperatures had cooled down some especially back
to the northwest and west where readings were down in the upper 50s and
lower 60s in some locations. Adjusted the hrly temperatures/dewpoints to
account for the current condition.
After teh fog this morning, the challenge will be the convective
potential this afternoon.
The frontal boundary has moved to the east west/fog and low clouds in
its wake. Some weak surface ridging was taking place per the 06z
analysis allowing from partial clearing above the low clouds and
fog that was in place at this time. The County Warning Area should see sunshine after
12z(8am)and the fog Burns off. It will remain quite humid today
west/daytime temperatures pushing through the 70s and lower 80s for
the northern 1/2 while central and downeast areas should see middle
80s. An upper level disturbance west/its associated surface trough will
move through the region this afternoon. There is enough middle level
forcing to kick off some showers and west/the atmosphere destabilizing.
Sb convective available potential energy are forecast to hit 800-1200 joules west/lifted indices dropping to -2.
Shear is there at the 0-6km layer(25kts). This would be enough to
allow for thunderstorms to develop. The lacking features to prevent
organized cells is that moisture appears to be limited to about
800mbs and 850-500mb lapse rates look marginal(6.0c/km).
Therefore, decided to go west/isolated thunderstorms and confined this to the
northern and western areas where best instability and forcing
For tonight, weak high pressure looks to ridge across the County Warning Area providing
mainly clear skies and cooler temperatures. Overnight lows expected
to be in the upper 50s to around 60. Concerned about some shallow
fog, but west/a light west wind forecast decided against it at this time.
The daycrew can assess this further today.
Short term /Saturday through Sunday/...
this period will begin with a large scale upper trough centered
across the Hudson Bay region. A series of upper level impulses
will traverse northern Maine within the mean flow both weekend
days, setting the stage for mainly afternoon showers and
The question is whether or not any storms on Saturday have the
potential to reach severe limits. The shear at 0-6 km is there
and is shown to be in the range of 35-45 kts. 500 mb-700 mb lapse rates
are also fairly impressive around 6.5-7.0 c/km, with freezing
levels expected to be around 10 kft. Limiting factor may be amount
of dry air aloft and marginal instability with convective available potential energy only around
500-900 j/kg. That being said, if storms develop they could tap
in to stronger winds aloft, and with aforementioned freezing
levels think hail would also be possible. Considering these
factors, decision was made to include enhanced wording for gusty
winds and small hail across northern and central areas for
Saturday afternoon. Also noted the new day two outlook from Storm Prediction Center
that has most of our region in marginal risk for severe.
Any ongoing showers and thunderstorms should quickly diminish
early Saturday evening as the upper impulse moves east and we lose
the heating. This will set the stage for partly cloudy skies
later Saturday night. Some patchy fog couldn't be ruled out
overnight with lows in the middle 50s to near 60 degrees.
Sunday should see another round of mainly afternoon showers and
thunderstorms. High temperatures both Saturday and Sunday will be
in the middle 70s north and upper 70s to near 80 down east, although
it will be a bit cooler on the coast.
Long term /Sunday night through Thursday/...
the unsettled weather pattern will continue into the long term. A
500 mb low centered near Hudson Bay at the beginning of the
period will migrate eastward by the end of the period. Will have
to keep the mention of the chance for showers through most of the
period. Couldn't rule out an isolated afternoon thunderstorm
either but will not mention at this time. Temperatures at the start of the
period will be near normal and then fall to below normal levels by
middle to late week as cooler air associated with the upper trough
settles across northern New England.
Aviation /12z Friday through Tuesday/...
near term: MVFR/IFR conditions in place at this time west/some fog and low
clouds. Conditions are expected to improve to VFR by middle morning
as drier works across the terminals. VFR looks to be a good bet
Short term: aviators can expect the chance for showers and
thunderstorms both Saturday and Sunday. Although we are expecting
VFR to prevail most of the daytime hours, there could be periods of
MVFR in any heavier showers or thunderstorms. Mainly VFR
conditions expected Monday and Tuesday in scattered afternoon
near term: fog could hamper navigation this morning. Drier air
working across the waters will allow for visibilities to improve. Winds
will increase out of the SW 10 to 15 kts. Expect seas to run 3 to
4 feet away from the intra-coastal zone.
Short term: wind/seas should generally remain below Small Craft
Advisory levels through the period.