Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Gray ME
717 am EDT sun Apr 26 2015
upper level low pressure will continue to wobble around over
Maine and the Canadian Maritimes through midweek....with the cool
and showery weather continuing. The low will finally shift east
allowing high pressure to build in for the second half of the work
week. Low pressure passes south of the area Thursday...with high
pressure building back in behind it.
Near term /today/...
will update the forecast to lower forecast high temperatures in
northern and central areas where cloud cover and showers will be
more widespread and should limit heating potential. No other
forecast changes anticipated yet.
pesky upper low remains over the western Atlantic with its axis
stretching back to the west over northern New England. The short
story is that the presence of this upper low will keep showery
weather in the forecast today through Monday. But there is more to
The broader upper can be thought of as a wheel... an Oblong shaped
one... but yes a wheel. This wheel is currently centered somewhere
off the southeast coast of Nova Scotia. Spokes within the wheel
rotate around the wheel center. Embedded shortwave troughs can be
thought of as the spokes of the wheel. However... not all spokes
are created equal. Some are larger or more potent than others. And
the strength and timing of each of these spokes determines whether
they will pass by without being noticed or whether they will cause
bigger snags (your picnic gets rained out).
The next spoke to come through our area will be arriving from the
north this morning moving out of Quebec and northern Maine. While
this spoke is fairly weak it will be moving through New Hampshire
and western Maine during the middle of the day... perfect timing
for triggering showers. The best chance of showers will be along
the New Hampshire and Maine state line.
Weak wind flow today will allow a sea breeze to develop near the
coast. As this pushes inland it will cut off surface based
instability but may not be enough to prevent places like Portland
from getting wet eventually.
Short term /tonight through Monday/...
continuing with the spokes analogy from the previous section...
the next spoke will move southwest through the forecast area
tonight... crossing through the center of the area after midnight
and nearing the Massachusetts line in the morning on Monday. This
spoke is stronger than the last one (strong enough to begin
pulling the center of the wheel itself back westward) but will be
moving through during the overnight hours when there is no diurnal
instability to work with. In spite of the lack of instability...
there will still likely be enough lift to trigger some showers
during the overnight period.
Finally the center of the wheel itself will spin southwest out of
Nova Scotia and into the Gulf of Maine during the day Monday. The
combination of lift and diurnal instability will lead to more
widespread showers especially over Maine and eastern New
Hampshire. With more showers expected it will also be a little bit
cooler on Monday than it will be today.
Long term /Monday night through Saturday/...
model guidance is good agreement on another shortwave pinwheeling
around the upper low pressure and crossing the forecast area
Monday evening. This looks to be the last gasp for control of our
weather...as the upper low pressure will begin to move out to sea
by middle week.
While we will tend towards rising heights and high pressure for
the latter half of the week...there is some uncertainty regarding
the evolution of the upper air pattern. Western ridge looks to
reestablish itself...helping to keep heights down along the East
Coast. This will support cyclogenesis along the eastern Seaboard.
How far north is low confidence at this time...because much will
depend on how southern shortwave interacts with northern stream
feature diving out of the Hudson Bay region. The GFS depicts
aggressive phasing and a much farther north solution...where the European model (ecmwf)
and CMC favor a delay and more S and east solution. At this time I
will favor a chance for showers with the passage of the northern
stream shortwave. Ridging tries to become more prominent heading
through the weekend.
Aviation /07z Sunday through Thursday/...
short term...generally VFR conditions but there is a better chance
for showers this afternoon over western Maine and eastern New
Hampshire. Expect an even better chance of showers and potentially
MVFR or IFR conditions at times on Monday.
Long term...generally VFR conditions expected to prevail in the
extended. Upper low pressure will remain nearby through about
Wednesday...so cannot rule out afternoon MVFR ceilings or
visibility in scattered showers. Given the small areal
coverage...any potential MVFR is fairly low confidence. Beyond
midweek will tend towards rising heights and high pressure...so
shower chances should diminish.
short term...weak pressure gradient will lead to very light winds
and nearly calm seas. Wind flow will likely be dominated by sea
breeze circulations especially today. By Monday low pressure will
be moving southwest into the Gulf of Maine and will allow
northerly winds to strengthen a bit by Monday evening.
Long term...Small Craft Advisory conditions are possible later in the work week
depending on how close to the Gulf of Maine low pressure tracks.
generally cool and showery weather will make dangerous fire
weather conditions unlikely for the next several days. Expect a
sea breeze along the coast this afternoon.