8/2/12 - Sunday's Severe Weather

Synopsis

High pressure moving off the coast will allow a south-westerly flow to funnel in a warm, moist air-mass over the coming days. A low-level jet at 5,000 ft (850 mb) will arrive on Sunday and boost the moisture content during the morning - shown on a map as a narrow corridor of enhance precipitable water (two standard deviations above average). Additionally, a surface trough enhanced by UL divergence will bring a line of showers during the afternoon, sunshine could then make a brief return, the new moisture will evaporate, and the air destabilizes again in time for the cold front's arrival during the evening hours. The right-entrance region of a jet streak will arrive at the same time as the front, enhancing the surface-based lifting mechanism from the cold front. Mostly, we're looking at damaging winds, heavy rain, and lightning. Small hail and a brief spin-up cannot be ruled out, but there's really no directional shear around. However, given enhanced moisture and the presence of two lifting mechanisms AND general model agreement between the European and GFS models, the NWS Storm Prediction Center has placed PA, NY, and parts of NJ, OH & western New England - including our area - under a 30% or higher probability for severe weather on Sunday, shown below.

The next outlook for Sunday will be posted on the SPC's outlook website at 3:30 AM Friday.

Models

Below is the output from the 18Z (2 PM EDT) run of the GFS (the American week-long forecast model). I've selected each of the six panels using AccuWeather Professional. From Left to Right then top to bottom: 500 mb Chart showing CCW rotation in two areas, a surface map showing heavy rain, the precipitable water map showing 1.8-2.2 inches in some spots, CAPE (instability), the 850 winds showing speeds of 35 kt (40 mph), and the jet stream winds.

Compare that to the same time using the 12Z (8 AM EDT) runs of the ECMWF (the European week-long forecast model).

The runs are very similar. The trend for CAPE has been downward and we're still three-and-a-half days away from the outbreak. Knowing that the forecast for hours 60-80 by the European model are pretty accurate, I'm going to monitor the trends over the next 24-48 hrs.

I also wanted to share this graphic with you, the GFS Ensembles' (probabilities) consensus vs. average precipitable water.

Orange indicates two standard deviations above average. Cause: the low-level jet.

Conclusion

Things look very good for severe storms on Sunday given that lifting, moisture, instability, and speed shear will be present. If things come together just right, watch out.

8/2/12: TD 5 Discussion

UPDATE (4 AM Friday): Now Tropical Storm Ernesto.

TD 5 (formerly Invest 99L) has formed Wednesday afternoon 800 mi (1300 km) SE of the Antillies. It has become less organized during the evening due to diurnal cooling, influx of dry air & shear on the north side. Here's the forecast scenarios, followed by a note about the intensity and then my forecast.

TRACK

Scenario 1: Heads WNW (280°). Landfall near Dominica Island around noon ET Friday and then hits Jamacia Sunday night. TS-force gusts, rainfall and an angry ocean affect the southern coasts in the northern Caribbean. (NHC Track, GFS model)

• Scenario 1A: Becomes a 75-mph hurricane near Jamacia. Hits Yucitan Peninsula Wed. AM; heads toward Gulf Coast. (Long-Rage GFS)
• Scenario 1B: Turns NE as it nears Jamacia. Hispanola, Cuba, Wrn Bahamas and eventually south FL affected. Will likely stay a TS if this occurs.

Scenario 2: Heads WNW (285-295°). Landfall near Guadeloupe Friday late afternoon and then Hispanola midnight Sat. night into Sunday. Turns NE: Cuba, Srn Bahamas, Straights of FL eventually affected. Puerto Rico could see TS conditions and better chance of heavy rain with this scenario. The storm could weaken after hitting Hispanola and Cuba. (Nrn NHC Track, European Model)

Scenario 3: Neads NE (295-310°). Landfall near Montserrat & Antigua Friday night, near Puerto Rico 24 hrs. later and then follows an Irene-like path through the SE Bahamas. After that, it could either turn N or turn back to the W. (Climatology, Canadian Model)

INTENSITY

I've discussed other intensity probabilities (i.e. becoming a hurricane) above. Just wanted to make it clear that this has a 95% chance of becoming the fifth named storm of the season - Ernesto. Most likely, that's sometime tomorrow (Thursday), but it could be Friday around 19Z at the latest.

MY CALL

Too Early To Tell, but a more northerly route than Scenario 1 makes sense right now. Don't hold me to that; we need dropsonde data. This route is central to 1 & 2 and gives me a few points if climatology is correct. I'll keep you updated, either here or on the Facebook Page.

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FORECASTER: BREEN

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