textproduct: Wakefield

WHAT HAS CHANGED

Confidence in rainfall continues to increase late Saturday into early Monday with moderate confidence in timing.

Isaias is now a hurricane.

KEY MESSAGES

1) Seasonable to slightly above average temperatures and dry conditions through Saturday.

2) Moisture and rain chances return Saturday night into early Monday.

DISCUSSION

As of 330 AM EDT Thursday...

KEY MESSAGE 1...Seasonable to slightly above average temperatures and dry conditions through Saturday.

GOES water vapor channels continue to show a split flow pattern early this morning. A northern stream trough is across the Great Lakes, and there is an associated cold front pushing across the Eastern Great Lakes. Meanwhile, the southern stream is evident in the cloud pattern to the N and NE of hurricane Isaias, with a trough in vicinity of TX. Mostly clear and not as cool locally ahead of the cold front, with temperatures mainly in the 50s to lower 60s, although some decoupling has occurred over interior S-central VA and interior NE NC where temperatures have dropped in the upper 40s.

The cold front slides across the region this afternoon. Mixing associated with the front and downslope flow should allow for high temperatures to rise above average and into the upper 70s to mid 80s. Mostly clear tonight behind to front with low temperatures ranging from the mid/upper 40s NW to the upper 50s/lower 60s SE. Temperatures trend down to near seasonal averages Friday and Saturday. Not necessarily impactful, but statistical guidance is on the order of 4-8F cooler than NBM guidance for lows late Friday night/early Saturday morning (lower 50s to lower 60s NBM vs. mid/upper 40s to upper 50s MOS). Dry conditions continue through Saturday. However, a gradual increase in high levels clouds is expected as upper level moisture spreads over the region well in advance of Isaias.

KEY MESSAGE 2...Moisture and rain chances return Saturday night into early Monday.

By Sunday and Monday, there is increasing confidence that moisture gets pulled northward and then pushed eastward across the region as Isaias makes landfall and merges with an upper trough in vicinity of the Mid-South. Confidence in timing is moderate as there is still some temporal differences in the model guidance. Of the 00z/08 deterministic global models and ensemble means, the EC AIFS remains to most progressive showing accumulating rainfall as early as the 18z Sat to 00z Sun time period. The bulk of the guidance (ECMWF, EPS, EC AIFS ens, CMC, and GEPS) are generally in the 06z-12z Sun timeframe for the onset of accumulating rainfall, and the GFS/GEFS are slightly slower and generally after 12z Sun. Incoming moisture will initially need to contend with a 1030mb high, which is progged to still be in vicinity of Cape Cod by 00z Sun, so a somewhat slower onset is logical. However, once deeper moisture arrives rain should begin quickly as 00z ensemble guidance continues to show PW values at or above 200% of normal along with a plume of instability surging N ahead of the trough. There is still spread between the 10th and 90th percentile QPF in the 00z/08 ensemble guidance. In general, the 10th percentile (reasonable low-end) accumulated precip through 00z Tuesday is 0.25-0.75", while the 90th percentile is on the order of 1.5-3.0". The 50th percentile (most likely case) shows a generalized 0.75-1.5" of rainfall, with the highest rainfall totals in the southern half of the area as there is a hint in the guidance of more enhanced rainfall along a coastal trough. The presence of a coastal trough would also suggest that an in situ wedge develops resulting in cooler temperatures and more stable conditions Sunday across the Piedmont. Deeper moisture erodes Sunday night into Monday. However, there are lingering PoPs as the upper trough swings across the region. Behind this system, drier air will filter back in later Monday through the middle of the week. Temperatures should remain near average late this weekend through the middle of next week.

AVIATION /12Z THURSDAY THROUGH MONDAY/

As of 640 AM EDT Thursday...

VFR conditions prevailed as of 12z with high pressure centered offshore. The wind was light out of the SW under a primarily clear sky aside from a band of cirrus S of ECG. VFR conditions are expected to continue today into tonight. The wind is expected to become WSW 5-10kt later this morning, and then shift to WNW as a dry cold front slides across the area. VFR tonight with a calm to very light generally N/NE wind.

Outlook: VFR conditions continue Friday and Saturday with increasing high clouds. The next chance for sub-VFR conditions arrives late Saturday into Sunday due to rain associated with the remnants of TC Isaias. Flight restrictions could linger into early Monday, followed by gradually improving conditions.

MARINE

As of 330 AM EDT Thursday...

Key Messages:

- Sub-SCA conditions prevail through Saturday.

- Small Craft Advisories are likely late this weekend into early next week as the remnant of Hurricane Isaias track through the central US, with high pressure well off the east coast.

S-SW winds have increased to 10-15 kt early this morning as a weak disturbance passes N of the area. Occasional gusts to 20 kt are expected to continue through sunrise, before dropping off later this morning. Weak sea/bay breezes develop this afternoon; otherwise, very benign on the waters. A weak cold front moves through overnight Thursday and northerly winds increase to 10-15 kt by early Friday morning. Lighter winds return Friday afternoon with high pressure again settling over the area. Seas are expected to stay in 2-3 ft range through most of Saturday. Waves in the bay will be 1-2 ft.

Hurricane Isaias has recently developed over the central Gulf and is forecast to make landfall in the western FL Panhandle early Saturday morning. While direct impacts are not anticipated locally, the interaction of the remnants of Isaias with an upper-level system is likely to lead to increased E-SE flow from Saturday night through Monday morning, with the peak in winds (20-25 kt w/ gusts to ~30 kt) occuring Sunday afternoon. Seas will also substantially increase to 5-8 ft and could be higher if winds trend up some, with 2-4 ft waves in the Chesapeake Bay. Regardless, Small Craft Advisories are likely during this time. The wind direction shifts to the W-NW Monday in the later half of Monday, while also gradually decreasing. A strong cold front could then pass through the area Tuesday into Wednesday, with elevated NW winds possible.

AKQ WATCHES/WARNINGS/ADVISORIES

MD...None. NC...None. VA...None. MARINE...None.