Analysis on Fri-Sat storm system and,,,,long range.
Posted: Thu Apr 24, 2003 9:00 am
Analysis on Fri-Sat storm and beyond ----------
Saturday- Our next rain feature in the Middle Atlantic and Northeastern US as a upper level feature moves toward us.
Quick synopsis- A mid to upper level storm system associated with the s/w trough will be on a neutral axis as it comes eastward. As it comes closer it will form a secondary low off Virginia. As the transfer occurs it will then ride northward bringing heavy rains and severe storms.
Forecast track- As the s/w trough takes a turn from a pos. tilt to a neutral axis it will then take a slight neg. tilt as it triggers off the secondary low. The upper level jet will be rather weak but it should be drawn northward phasing with the northern stream therefore allowing a track northward from VA.
Severe WX threat for Friday-
I like the SPC threat area and in brief here it is:
Mississippi, Alabama, Georgia, Tennessee, Western NC, and Western South Carolina.
As the cold front progresses eastward associated with our upper level low will see some nice dynamics come together to produce some severe weather. Capping will be very weak and will easily erode with sufficient lift and strong instability. The question right now is after the warm front passes with moderate-heavy rains,,does the sun come out and we get some nice adiabatic and diurnal heating leading to nice thermodynamic instability. Or does it remain cloudy,,,and it will still lead to a nice outbreak of severe wx but the sunshine if any will only contribute to the threat. Lower levels are also looking ominous with strong LLJ between 35-45KTS with strong WAA and isentropic lifting. Upward Vertical Velocities will be very strong as the vortmax associated with our upper level low will extend moderate to significant PVA down to the threaten areas with a nice MLJ with wind speeds of 45-55KTS helping the growth of cumulous clouds. Upper level flow is in the weak range as the jet max will be around 60KTS. With the absence of a strong flow aloft all other levels will have strong wind fields. With that said there will also be nice speed shear and vertical shear to help in the aid of development with supercells. Directional shear will almost be non existent if models are right, but even with a little directional shear the steep mid level lapse rates could make for strong rotating updrafts and tornadoes will me a major threat, but right now isolated tornadoes, large hail, and damaging winds are the threats. Some model guidance shows lift indices as low as -9C. SFC dewpoints will be very high, mostly in the range of 60-65°F, however some areas especially in the southern sections of the threat areas were dew points could pass the 70°F mark. H8 dew points also high in the range of 12-15°C. Southern areas of the threat zone will again see the highest SBCAPE values between 2500-3500j/kg.
Saturday the severe threat will move into the Carolina's and possibly Southern Virginia. More detail on this later.
------------Back to rain event-------------
As the secondary low forms there will be a wide area of nice low level and surface speed convergence with the diffluent flow aloft. The low level and surface convergence will lead to upr air divergence and will intensify the area of low pressure. But the deepening process will be gradually and not rapidly. UVV's also will be strong as we get some nice PVA and where the low level convergence is strong. We will also see some nice overrunning precipitation as a cyclone form the north and a mid level ridge moves in from the West will produce some CAA and with the strong waa we could see a nice widespread area of rain, and RH fields will also be in support of a widespread event as values will range from 90-100%.
For rain totals I'm leaning towards the higher totals and right now the best guess would be probably (for major cities DCA-NYC) between 1.0-1.50". Rainfall amounts will probably be highest inn the major cities rather than the other areas to the south, with the exception of the areas who will receive strong to severe storms which will enhance totals. The reasoning for the higher totals to the north of Central VA is once our coastal gets kicking will see an increase of ATL advection. Severe threat for Saturday could extend as far nroth as extreme S. DE. And wouldn't be suprised to see isolated strong to severe cell north of S. DE. As the Low pressure system moves eastward it will run into some ridging in the ATL and will create a strong pressure gradient leading to strong winds particularly from SC-E NC-E VA. And as the Low moves on north will see a strong easterly flow out ahead fof it leading to some tidal flooding. Also with the -NAO sig. will see a north to south orientation making the track of the low more certain with northerly movement once it develops.
LONG RANGE-----8-15 days-------
With good ensemble agreement will see the return of the positive phase of the PNA with the nice ridging over the Western States and troughiness in the Eastern US. With the AO forecast by ensemble agreement will see a weak-mod negative meaning nice ridging over the poles and perhaps another period of below average temps. But we still have time to monitor it and it won't be too drastic. Like always GFS shows a slow moving cold front in the day 8-10 range, but looking at the NAO sig it looks positive therefore more of a W-E orientation therefore a normal-quick moving front. The GFS shows a cut off high pressure system in the ATL which is looking like an outlier as ensembles clearly go against it.
Saturday- Our next rain feature in the Middle Atlantic and Northeastern US as a upper level feature moves toward us.
Quick synopsis- A mid to upper level storm system associated with the s/w trough will be on a neutral axis as it comes eastward. As it comes closer it will form a secondary low off Virginia. As the transfer occurs it will then ride northward bringing heavy rains and severe storms.
Forecast track- As the s/w trough takes a turn from a pos. tilt to a neutral axis it will then take a slight neg. tilt as it triggers off the secondary low. The upper level jet will be rather weak but it should be drawn northward phasing with the northern stream therefore allowing a track northward from VA.
Severe WX threat for Friday-
I like the SPC threat area and in brief here it is:
Mississippi, Alabama, Georgia, Tennessee, Western NC, and Western South Carolina.
As the cold front progresses eastward associated with our upper level low will see some nice dynamics come together to produce some severe weather. Capping will be very weak and will easily erode with sufficient lift and strong instability. The question right now is after the warm front passes with moderate-heavy rains,,does the sun come out and we get some nice adiabatic and diurnal heating leading to nice thermodynamic instability. Or does it remain cloudy,,,and it will still lead to a nice outbreak of severe wx but the sunshine if any will only contribute to the threat. Lower levels are also looking ominous with strong LLJ between 35-45KTS with strong WAA and isentropic lifting. Upward Vertical Velocities will be very strong as the vortmax associated with our upper level low will extend moderate to significant PVA down to the threaten areas with a nice MLJ with wind speeds of 45-55KTS helping the growth of cumulous clouds. Upper level flow is in the weak range as the jet max will be around 60KTS. With the absence of a strong flow aloft all other levels will have strong wind fields. With that said there will also be nice speed shear and vertical shear to help in the aid of development with supercells. Directional shear will almost be non existent if models are right, but even with a little directional shear the steep mid level lapse rates could make for strong rotating updrafts and tornadoes will me a major threat, but right now isolated tornadoes, large hail, and damaging winds are the threats. Some model guidance shows lift indices as low as -9C. SFC dewpoints will be very high, mostly in the range of 60-65°F, however some areas especially in the southern sections of the threat areas were dew points could pass the 70°F mark. H8 dew points also high in the range of 12-15°C. Southern areas of the threat zone will again see the highest SBCAPE values between 2500-3500j/kg.
Saturday the severe threat will move into the Carolina's and possibly Southern Virginia. More detail on this later.
------------Back to rain event-------------
As the secondary low forms there will be a wide area of nice low level and surface speed convergence with the diffluent flow aloft. The low level and surface convergence will lead to upr air divergence and will intensify the area of low pressure. But the deepening process will be gradually and not rapidly. UVV's also will be strong as we get some nice PVA and where the low level convergence is strong. We will also see some nice overrunning precipitation as a cyclone form the north and a mid level ridge moves in from the West will produce some CAA and with the strong waa we could see a nice widespread area of rain, and RH fields will also be in support of a widespread event as values will range from 90-100%.
For rain totals I'm leaning towards the higher totals and right now the best guess would be probably (for major cities DCA-NYC) between 1.0-1.50". Rainfall amounts will probably be highest inn the major cities rather than the other areas to the south, with the exception of the areas who will receive strong to severe storms which will enhance totals. The reasoning for the higher totals to the north of Central VA is once our coastal gets kicking will see an increase of ATL advection. Severe threat for Saturday could extend as far nroth as extreme S. DE. And wouldn't be suprised to see isolated strong to severe cell north of S. DE. As the Low pressure system moves eastward it will run into some ridging in the ATL and will create a strong pressure gradient leading to strong winds particularly from SC-E NC-E VA. And as the Low moves on north will see a strong easterly flow out ahead fof it leading to some tidal flooding. Also with the -NAO sig. will see a north to south orientation making the track of the low more certain with northerly movement once it develops.
LONG RANGE-----8-15 days-------
With good ensemble agreement will see the return of the positive phase of the PNA with the nice ridging over the Western States and troughiness in the Eastern US. With the AO forecast by ensemble agreement will see a weak-mod negative meaning nice ridging over the poles and perhaps another period of below average temps. But we still have time to monitor it and it won't be too drastic. Like always GFS shows a slow moving cold front in the day 8-10 range, but looking at the NAO sig it looks positive therefore more of a W-E orientation therefore a normal-quick moving front. The GFS shows a cut off high pressure system in the ATL which is looking like an outlier as ensembles clearly go against it.
8)
