Area Forecast Discussion
Issued by NWS Albany, NY

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FXUS61 KALY 262329
AFDALY

Area Forecast Discussion
National Weather Service Albany NY
729 PM EDT Wed Jul 26 2017

.SYNOPSIS...
An approaching frontal system will bring some showers
and thunderstorms late tonight into Thursday. Dry weather is
expected for most of the area Friday into the weekend; however, some
rainfall is possible across far southern areas as low pressure
passes by to the south. Temperatures will remain a bit cooler than
normal.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
Satellite, radar and other trends in data support the
current forecast through this evening. Precipitation looks to
be a little behind schedule and have adjusted some of the sky
cover and rain chances between midnight and daybreak. The
previous AFD has a few more details and is below...

High clouds will continue to thicken and lower tonight as
midlevel heights gradually fall and increased midlevel theta-e
washes into the area. Weak ascent provided by the height falls
and moisture advection will likely allow for some showers to
break out, especially north of the Capital District, but will
have some dry air to contend with. So have gone with a more
gradual increase in PoPs from north to south tonight, with best
chances across the north after 06Z. Lows are expected to be
milder than the past couple of nights with light southerly flow
and increasing cloud cover. Coolest temps may be found across
the far south where it will take longest to cloud up.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH SATURDAY/...
Weak midlevel height falls will continue Thursday as a broad
upper trough sweeps into the region. A weak surface cold front,
tied to a surface low in northern Quebec, will also push into
the forecast area. A seasonably moist airmass will wash into the
region from the west ahead of the front, characterized by PWATs
in the 1.50-1.75 inch range. Model consensus suggests areas of
rainfall associated with theta-e advection and midlevel height
falls to expand southward and impact the northern 2/3 of our
forecast area between 12-18Z. The expected cloud cover and
rainfall will limit destabilization, with even the typically
bullish NAM struggling to generate MLCAPE values of 1000 J/kg
during the afternoon despite dewpoints rising into the mid to
upper 60s. The GFS generates little to no surface-based
instability for the majority of the forecast area. Even if some
instability develops with muted surface heating, convergence
along the front appears to weaken by the afternoon hours as the
front washes out. So, while an isolated stronger storm cannot be
ruled out, especially in the afternoon, an organized severe
weather event appears unlikely. Locally heavy rainfall will be a
threat, given midlevel winds nearly parallel to low-level
boundary, along with the moist atmosphere. However, uncertainty
with respect to convective coverage as well as fairly fast
precip motion should mitigate the threat somewhat. Have gone
with a blend of the cooler MAV and warmer MET guidance for
highs, yielding upper 60s to upper 70s.

Lingering showers and isolated thunderstorms will remain
possible early Thursday night over southern portions of the area
before drier air builds in on northwest flow as high pressure
begins to nose in. Can`t rule out some fog if clearing develops,
especially in areas that see rainfall Thursday. Lows near to a
bit above normal in the upper 50s to upper 60s.

Friday into Saturday, a vigorous, compact upper low will move from
the Western Great Lakes to the Mid-Atlantic coast. A seasonably
strong surface low will develop ahead of this feature in the
lee of the Mid-Atlantic Appalachians and track into the Atlantic
south of Cape Cod by 00Z Sunday. Still considerable model
spread with the track of the surface low and the northern extent
of its associated precip shield. Consensus does favor the
majority of the precipitation remaining south of the local
forecast area; however, the northern fringe could reach our
southern two tiers of counties Friday afternoon into Friday
night. Heavy rainfall threat at this time looks to set up south
of our forecast area, but a few outlying SREF/GEFS members
indicate heavy rainfall extending into our far southern
counties. However, the trend in the GEFS has been drier. At this
point, will continue to toe the line with chance PoPs until
better model consensus can be reached. For the remainder of the
forecast area, relatively pleasant weather can be expected, with
highs in the upper 60s to upper 70s (a bit below normal). Have
sided with the cooler MET MOS for highs on Friday given that the
GFS appears to be a slow outlier with the system, providing
ample sunshine and warmer temps for much of our forecast area.
Could be a bit breezy Saturday for this time of year on the back
side of the low. Northerly winds will allow H850 CAA to occur,
dropping temps to the upper single digits Celsius.

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
We finally have a stretch of pleasant weather in store for us in the
long term. Temperatures start out near normal but rise to the above
normal side of the spectrum with each passing day and precipitation
looks to be below normal due weak forcing and paltry upper level
moisture, based on the current model suite.

Sensible weather on Sunday is contingent on just how fast a potent
coastal low off the NJ/S. New England coast develops and departs
over the weekend. At this time, the low should be well south of our
CWA with high pressure anchored over NY/northern New England keeping
most of our regions dry under northerly winds. Have left cloudier
skies/cooler temperatures (upper 70s) for the southern/eastern tier
areas and clearer skies/slightly warmer temperatures (near 80)
elsewhere based on this thinking.

By Monday, high pressure should be in control over our CWA with
temperatures warming to normal everywhere. Tuesday the high shifts
further eastward with a weak trough dipping out of Canada into
northern New England. Current guidance shows weak forcing with the
base of the trough barely reaching our CWA with little in the way of
H700 moisture. Thus, have only placed slight chance POPs in our CWA,
mainly northern/elevated area. With the high situated further east
giving us more westerly flow, CAPE values look to increase above
500J/kg so placed slight chance thunder as well for Tuesday.

Wednesday - Thursday turn warmer and more humid as high pressure
shifts even further eastward and southwesterly flow takes over the
region. Temperatures should rise into the mid 80s throughout the CWA
with dew points returning to the 60s. The H500 flow shows zonal/weak
ridging over the Northeast during this period ahead of the next,
stronger trough approaching late in the week. On Wednesday, guidance
suggests very weak shortwaves embedded in the upper level
atmospheric flow which means a few isolated afternoon storms are not
ruled out. The aforementioned trough should drag a cold front late
Thursday/Thursday evening and guidance does show a decent thermal
gradient so that time period has a higher threat for
showers/storms.

&&

.AVIATION /23Z WEDNESDAY THROUGH MONDAY/...
High clouds are thickening and lowering tonight and rain in the
Great Lakes is slowly building south ahead of a front dropping
out of Canada. Have estimated the timing of the onset of
showers from north to south, from KGFL to KALB/KPSF and KPOU
between 06Z and 10Z when VCSH has been included at KGFL, KALB
and KPSF.

KPOU will see showers later Thursday morning, perhaps in the
15Z time frame when VCSH was added there. By 11Z-14Z KGFL, KALB
and KPSF should see intervals of showers with MVFR visibilities
and ceilings. Some intervals of IFR visibilities and near IFR
ceilings during the late morning and early afternoon.

There could also be scattered thunderstorms over the entire
region but due to the scattered nature, not indicating thunder
in the TAFs until radar trends allow for more precise inclusion
in TAFs on Thursday. KPOU will see intervals of near MVFR
visibilities but ceilings should fall into the MVFR range much
of Thursday afternoon. The steadier showers should exit south
of KGFL, KALB and KPSF by 18Z- 20Z but VCSH included through the
afternoon due to timing uncertainties. VCSH also included at
KPOU for the same reasons.

South winds at less than 10 Kt will remain light through tonight
and Thursday morning. Winds shift to southwest at less than 10
Kt Thursday afternoon.


Outlook...

Thursday Night: Moderate Operational Impact. Chance of SHRA...TSRA.
Friday: No Operational Impact. NO SIG WX.
Friday Night: No Operational Impact. NO SIG WX.
Saturday: No Operational Impact. NO SIG WX.
Saturday Night: No Operational Impact. NO SIG WX.
Sunday: No Operational Impact. NO SIG WX.
Sunday Night: No Operational Impact. NO SIG WX.
Monday: No Operational Impact. NO SIG WX.

&&

.FIRE WEATHER...
Showers and thunderstorms will become likely once again
Thursday, especially north of the Mid-Hudson Valley. Drier
weather takes hold Friday into the weekend, although periods of
rainfall may impact areas south of the Capital District Friday
and Friday night. Temperatures will come up short of normal
values. RH values will bottom out in the 55 to 65 percent range
Friday, and become drier Saturday in the 40 to 50 percent
range. Northerly winds may become a bit gusty Saturday to near
25 mph.

&&

.HYDROLOGY...
Periods of showers and thunderstorms are expected Thursday into
early Thursday night. Basin average rainfall is expected to
remain below a half inch; however, locally heavy rainfall will
be possible in any thunderstorm. Isolated urban and poor-
drainage flooding cannot be ruled out in areas that see repeated
rounds of rainfall.

Conditions will generally be dry Friday into the weekend, but
some periods of rainfall may impact areas south of the Capital
District Friday afternoon into Friday night. Latest indications
are that amounts will remain on the light side; however, there
is a low probability that the associated storm track will shift
north and bring more of a soaking rain to southern portions of
the forecast area.

For details on specific area rivers and lakes, including
observed and forecast river stages and lake elevations, please
visit the Advanced Hydrologic Prediction Service /AHPS/ graphs
on our website.

&&

.ALY WATCHES/WARNINGS/ADVISORIES...
CT...None.
NY...None.
MA...None.
VT...None.

&&

$$
SYNOPSIS...Thompson
NEAR TERM...NAS/Thompson
SHORT TERM...Thompson
LONG TERM...Speciale
AVIATION...NAS
FIRE WEATHER...Thompson
HYDROLOGY...Thompson


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