Focus Area: Pennsylvania
Synoptic features:
500 mb: Negatively tilted trough
250 mb: Diffluent pattern over SErn PA from splitting branches of the jet stream.
Surface system: occluding mid-latitude cyclone
Mesoscale parameters:
Instability: LOW – SOME PRESENT
Shear: MODERATE – NOT OVERWHELMING
Conclusions:
A squall line associated with the cold front is still the best chance of seeing severe weather across the southern two-thirds of PA. Winds to 70 mph and hail to an inch thick possible. Ahead of the squall line, multi-cell clusters with some isolated super-cell activity are possible due to abundant shear and some instability, but cloud cover lowers probabilities substantially over PA. More breaks in the cloud cover between 10 am and 3 pm are necessary for this scenario to play-out, but there’s a chance of two-inch thick hail and a few isolated yet weak tornadoes if storms form ahead of the squall line.
Other notes:
Thunderstorms will be slow-movers (15-25 kts) with 1-2 inches of precipitable water, increasing the chance of flash flooding from storms. Any storm is also capable of producing dangerous cloud-to-ground lightning.
I will continue to monitor the models and trends. For those of you in the mid-Atlantic States, there is more instability but less shear, so expect clusters of cells and a stronger squall line to organize.
Next entry: Tomorrow by 7:30 AM hopefully.
Acknowledgements: SPC Convective Outlooks and discussions with Steve Simon (grad student) & Scott Sieron (honors ugrad student).
Thursday, May 31, 2012
Wednesday, May 30, 2012
5/30/12 - Friday Severe Weather Scenario
Focus Area: Southern half to two-thirds of PA
Primary threats: Early chance of supercells and multi-cell clusters in the SE quadrant of PA. A squall line will form along the cold front during the afternoon. Timing of the storms will be very similar to yesterday's severe weather.
Instability: Lower than Tuesday
- SB CAPE: About 1000 J/kg (Tuesday's storms developed with similar amount of SB CAPE).
- ML CAPE: About 500 J/kg (Tuesday's storms had up to 2000 J/kg).
Shear: Very high. There was no shear on Tuesday
- 0-1 km SR Helicity = 200-400 m^2 / s^2
- 0-3 km SR Helicity = 250-500 m^2 / s^2
- Jet Stream Presence: CONFIRMED
Synopsis:
The set-up will be similar to Tuesday with notable exceptions.
1. Instability will decrease due to the passage of the warm front occurring on Thursday night and its proximity to the PA-NY border. There will be enough to touch off storms in similar fashion.
2. Shear will be dramatically high since we are near the triple point and close to a jet streak.
3. We're in an area of enhanced upper-level divergence from the jet streak. An upper-level trough at 500 mb (~18,000 ft) will tilt into the low, providing some occlusion -> necessary for severe storms.
Further details (probabilities) to come on Thursday and Thursday night. The SPC has issued a slight risk but if models runs over the next two daily cycles are consistent or the front is expected to move through a bit earlier, a moderate risk of severe could be necessary in one of tomorrow's Day 2 outlooks.
Primary threats: Early chance of supercells and multi-cell clusters in the SE quadrant of PA. A squall line will form along the cold front during the afternoon. Timing of the storms will be very similar to yesterday's severe weather.
Instability: Lower than Tuesday
- SB CAPE: About 1000 J/kg (Tuesday's storms developed with similar amount of SB CAPE).
- ML CAPE: About 500 J/kg (Tuesday's storms had up to 2000 J/kg).
Shear: Very high. There was no shear on Tuesday
- 0-1 km SR Helicity = 200-400 m^2 / s^2
- 0-3 km SR Helicity = 250-500 m^2 / s^2
- Jet Stream Presence: CONFIRMED
Synopsis:
The set-up will be similar to Tuesday with notable exceptions.
1. Instability will decrease due to the passage of the warm front occurring on Thursday night and its proximity to the PA-NY border. There will be enough to touch off storms in similar fashion.
2. Shear will be dramatically high since we are near the triple point and close to a jet streak.
3. We're in an area of enhanced upper-level divergence from the jet streak. An upper-level trough at 500 mb (~18,000 ft) will tilt into the low, providing some occlusion -> necessary for severe storms.
Further details (probabilities) to come on Thursday and Thursday night. The SPC has issued a slight risk but if models runs over the next two daily cycles are consistent or the front is expected to move through a bit earlier, a moderate risk of severe could be necessary in one of tomorrow's Day 2 outlooks.
Sunday, May 6, 2012
NE US Rainfall - May 7-11, 2012
Short term outlook for the Northeast States.
Flooding rainfall expected over the next five days. Models suggest that rainfall amounts of one to four inches could fall over much of the Northeast US. Thunderstorms on Tuesday and Wednesday could produce heavy rainfall, enhancing totals where storms occur. Also, the SPC is monitoring the region for strong to severe thunderstorms. If you live near rivers or in other areas vulnerable to flooding, take precautions today.
Flooding rainfall expected over the next five days. Models suggest that rainfall amounts of one to four inches could fall over much of the Northeast US. Thunderstorms on Tuesday and Wednesday could produce heavy rainfall, enhancing totals where storms occur. Also, the SPC is monitoring the region for strong to severe thunderstorms. If you live near rivers or in other areas vulnerable to flooding, take precautions today.
Friday, May 4, 2012
PA T'storm Potential Update - 5.4.12
Fact: Today is the fifth anniversary of the Greensburg, KS tornado that killed 11 people and almost wiped that town off the map.
The following short-term forecast is for central PA.
Slight risk of severe t'storms for most of the Commonwealth.
Sunny, very warm and muggy between now and 2 PM with highs into the 80s dew points in the 60s. Thunderstorms develop after 3 PM and evolve into clusters just like the past few days. Primary threats: rainfall rates to 0.8"/hr and lightning. Other threats: wind gusts (mainly 25-40 mph with isolated speeds to 60 mph) & small hail (pea- to half-dollar-size at the biggest).
Acknowledgements: I was at the NWS in State College from 8:30-10 AM and listened to their brief discussion, which was the motivation for writing this.
Probability of a Watch: less than 15%.
Disclaimer: The forecasts I posted this week and other weeks in the future are my own and not the official forecasts/ opinions of other sources I consult. I will give credit to those sources for the information they provide.
The following short-term forecast is for central PA.
Slight risk of severe t'storms for most of the Commonwealth.
Sunny, very warm and muggy between now and 2 PM with highs into the 80s dew points in the 60s. Thunderstorms develop after 3 PM and evolve into clusters just like the past few days. Primary threats: rainfall rates to 0.8"/hr and lightning. Other threats: wind gusts (mainly 25-40 mph with isolated speeds to 60 mph) & small hail (pea- to half-dollar-size at the biggest).
Acknowledgements: I was at the NWS in State College from 8:30-10 AM and listened to their brief discussion, which was the motivation for writing this.
Probability of a Watch: less than 15%.
Disclaimer: The forecasts I posted this week and other weeks in the future are my own and not the official forecasts/ opinions of other sources I consult. I will give credit to those sources for the information they provide.
Tuesday, May 1, 2012
Centre Region Forecast - May 1, 2012
Focus City: State College, PA
Seven-Day Forecast Discussion:
A weak mid-latitude cyclone that touched off isolated thunderstorms last night is currently moving across the Commonwealth today. Cloud cover will begin decreasing. Overnight, high pressure will slide eastward over southern New England, which will be in control of our weather for about 24 hours. A cluster of weak upper-level disturbances (500-mb vorticity maxima) will "bring" a trough (axis of low pressure) northward (showers & storms possible), pushing the high pressure system off the New England coast - all that to occur tomorrow night into Thurs. AM. The trough will settle over southern Upstate NY (near I-90) on Thursday. While near the trough axis (and its zone of mass convergence) on Thursday and Friday, sunny skies may lead to nearby pop-up thunderstorms (the "self-destructing sunshine" effect). On Thursday, a backdoor cold front will head into eastern PA and start to weaken as it heads into the state. But the temperatures east and west of the BDF will be quite different. Two cold fronts will make their way from west to east: one on Friday and another on Saturday... which could lead to showers and thunderstorms each day. Strong storms, while only a remote possibility of occurring, cannot be ruled out entirely. High pressure will move into the area on Sunday, settling over the Northeast US on Monday.
Acknowledgements: I consulted the official discussion from the NWS in State College.
Temperatures: Seventies today and eighties for the second half of the week. Back to the low-to-mid-60s early next week.
Long-term Briefing:
A weak system could be nearby during the middle of next week. Two more systems that look pretty strong could arrive next weekend and in the middle of the following week respectively. In between, look for passing high pressures systems to clear the air for a couple of days.
Seven-Day Forecast Discussion:
A weak mid-latitude cyclone that touched off isolated thunderstorms last night is currently moving across the Commonwealth today. Cloud cover will begin decreasing. Overnight, high pressure will slide eastward over southern New England, which will be in control of our weather for about 24 hours. A cluster of weak upper-level disturbances (500-mb vorticity maxima) will "bring" a trough (axis of low pressure) northward (showers & storms possible), pushing the high pressure system off the New England coast - all that to occur tomorrow night into Thurs. AM. The trough will settle over southern Upstate NY (near I-90) on Thursday. While near the trough axis (and its zone of mass convergence) on Thursday and Friday, sunny skies may lead to nearby pop-up thunderstorms (the "self-destructing sunshine" effect). On Thursday, a backdoor cold front will head into eastern PA and start to weaken as it heads into the state. But the temperatures east and west of the BDF will be quite different. Two cold fronts will make their way from west to east: one on Friday and another on Saturday... which could lead to showers and thunderstorms each day. Strong storms, while only a remote possibility of occurring, cannot be ruled out entirely. High pressure will move into the area on Sunday, settling over the Northeast US on Monday.
Acknowledgements: I consulted the official discussion from the NWS in State College.
Temperatures: Seventies today and eighties for the second half of the week. Back to the low-to-mid-60s early next week.
Long-term Briefing:
A weak system could be nearby during the middle of next week. Two more systems that look pretty strong could arrive next weekend and in the middle of the following week respectively. In between, look for passing high pressures systems to clear the air for a couple of days.
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