IWIC US discussion

[Allexpert Mike]

US Discussion updated 2:25 EDT, Wednesday June 18, 2003
Will start off this forecast with the activity along two main fronts across the CONUS. First off, were currently in a +PNA pattern with ridging dominating out in the Western US and troughiness in the Eastern US. This pattern will reverse as we head into day 3 and on as troughiness moves into the Western US with s/w trough dipping down into Southern CA were a ULL will come ashore. As we go through time the trough will deepen with strong upper level jet streak on the west side of the trough. With the reversal from ridge to trough out West it won't mean trough to ridge for the Eastern US as ULL over the Midwest states will keep troughiness in along with a prolong threat for on/off rain across the Northern Middle Atlantic and Northeast. With troughiness on both sides of the US will see a strong omega sig. ridge in the Center of the Nation. 5+ day forecast is at the bottom.

1-3 day forecast

Northeast-

Upper level low will sit right over the Midwest and will slowly push eastward. With strong ULJ rounding the base of the trough along with strong upper level jet streak over the Northern Middle Atlantic into the Northeast over E. PA/NJ will see strong jet support enhancing precipitation along with the vort lobe advecting eastward increasing PVA. With PVA increasing will see stronger uplifting and with strong low-level convergence, primarily along coastal areas, will see more in the way of uplifting increasing the upward vertical velocities to a moderate to strong level. This front will slowly push off the coast by day 3 due to the MLJ having a parallel flow to the front.

Southeast-

Severe weather will be a threat across the Southeastern US with this slow moving front. With mid level flow parallel to the front will see it move very slowly before moving out of here. Remainder of today will see strong upper level energy with ULL helping to support some heavy showers and storms along the frontal boundary. Open surface waves of low pressure will ride along the front brining a increase in intensity and coverage to the rainfall, these sfc waves are closed at H8 and as we head into later tonight will see these sfc waves close out at sfc. As we head through next 3 days it will stick with us with strong s/w energy riding along the stalled frontal boundary.

Southwestern US-

Will keep this short and sweet. It will be hot and dry over the Southwestern CONUS the next 1-3 days and beyond. The heaviest rains will stay well to our north leaving the Southwestern CONUS in the dry and hot weather.

Northwestern US-

It will be relatively quiet until day 3 were dynamics will come together to produce some severe weather across the region in Montana to Idaho down towards Nevada and Utah. Strong MLJ structure shows strong and sufficient deep layer shear ranging form 35-40KTS. Strong upper level jet streak on west side of the trough shows that the trough will continue to deepen as we head into day 4. Strong jet diffluence and vertical shear profiles may team up and create the potential for some super cells to form. Northern stream vort. along with H5 s/w energy will see increase in PVA and with strong MLJ will see moderate vorticity advection. Strong low-level speed convergence will see moderate to strong UVV's across the threaten area. H5 cold pool with H5 temps ranging form -14 to -16C will keep SBCAPE values down and will aid in threat for severe weather. Instability will be moderate with the moderate waa and TT index ranging from 45-50dC. SBCAPE values will be very low ranging from 500-1000j/kg over the threaten area. Strong speed shear along with strong deep layer shear will help increase the 0-3km helicity values to around 350m^2/s^2 across Montana. Elsewhere 0-3km helicity values will range form 150-250m^2/s^2. Directional shear won't be impressive at all with winds backing with height with SW flow at sfc and S flow aloft. Main threats in this area will be for tornadoes and large hail up to 2.0" in diameter. Damaging winds will be confined to the stronger cells with high moisture fields in all levels leading to minimal dew point depressions.

5+ day outlook for areas of interest

I’m not backing away from my thinking for a major rainfall in the Upper Midwest around Minnesota into Wisconsin. Reasoning is a rex blocking sig. will set up and set up will be favorable for storm development in the trough and models aren’t showing that right now. However I’m going to go against models for the time being and say that storm will develop and the trough will take a neg. tilt leading to all sorts of favorable dynamics with strong jet diffluence downstream and upper air divergence leading to rapid development with strong jet. The low will then move very slowly as it will be stuck in a rex blocking sig. and as a result continue to bring prolong rainfall to this region. There’s still lots of uncertainty with this system so will continue to monitor it in the 6-10 day range.

In the Northeast and Midwest will see another cold front pass through brining threat for showers and storms some of which could be severe. The better chance for severe weather will be in the Midwest as models show upper level ridging over the Eastern US, which will keep from anything widespread developing. This front will stall out in the Southeastern US back towards the Southern Plains as the –PNA pattern will dominate and as a result will see a deep trough out West with main ULJ being so far north that there will be nothing to push this front out so we could be talking another prolong rain threat for areas of the Southeastern US. With GOM advection will see the Gulf moisture add some fuel to these showers and storms which will be slow moving and this could result in flooding, will continue to monitor these situations as they come closer.

EPO and WPO did reach the positive phase this past week and we did see some hot weather over here in the Eastern US with high temps well into the 80’s. As we look at longer-range models it looks like a cut off high will position itself in the Gulf of Alaska ruining the +EPO and as a result the EPO will become even more negative in the weeks ahead. WPO will also take a negative trend with the new ECWMF showing the neg. phase dominating. The NAO will be in a positive phase along with the AO with low height dominating over the Polar regions along with the low heights dominating over Greenland.

Forecaster: Mike D.





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[grentz7721]

Mike, According to the TWC Forecast, The Southeast is not likely to get
Severe Weather. As in Fact, I think there is No Boundary Front
that will not cause weather.

[Guest]

Im not a 100% in agreement with the SE as well...............Yes i do think the front will die down somewhere in the southeast probably near the gulfcoast however im not so sure about what kind of severe weather you are talking about.............1Do you mean severe as in widespread with Large hail,Damaging winds and even tornadoes? 2.Or do you mean severe as in very heavy rainfall adding to the flooding problems that they have been having as of late??????? My take on it is that there will be quite a few thunderstorms (Some beeing severe as in example one) And a better shot at the type of severe threat as in example 2...........The real question is how far south will that front get....................Which at this point i cant even say.............My best guess would be somewhere along the gulfcoast states...................And we still cant rule out the afternoon storms in the inter mountain west that they have been seeing this past week.............Anyways thats my take on it.........