Mesohigh Heavy Rain Events

MESOHIGH 850 mb

Approximately half the time, these mesohigh events occur east of a slow moving frontal band where it is intersected by the outflow boundary.† Surface dewpoint are usually above 70 deg F south of both the front and outflow boundary.† The typical surface, 850 and 500 mb patterns for mesohigh events is shown below

850 mb dew points south of the boundary are generally in the mid teens leading to K-indices in the upper 30s.† The greatest threat area of excessive rainfall is usually found in the southern or western portions of the mesohigh.† Essentially, mesohigh events and frontal events are the same.† The scale of development is the main difference.† An old heavy precipitation forecasting admonition is bewaring of east-west frontal or thermal boundaries, they are dangerous.

Mesohigh events have a propensity to develop near the axis of the upper level ridge where the winds at mid and upper levels are fairly weak and the shear is relatively weak.

What makes the Frontal and Meso-high type patterns identified by Maddox et al. particularly favorable for heavy rains.† Letís look at them from an ingredients based methodology.† The article notes that the airmass is unstable with anomalously high precipitable water (PW) for both types.† The combination should produce adequate vertical moisture flux into the cloud to produce heavy rains.†† Both composite shows an axis of higher 850 winds that are also coincident with high 850 dewpoints suggesting strong there is maximum of horizontal moisture flux feeding into the thermal boundary.† This is a plus for trying to sustain the vertical moisture flux into the system.

 

The low level jet is often located to the southwest of the boundary which helps maintain low level convergence on the southwest side of any convective system than develops.† The mean flow is westerly so the MCS often develops into a Parker and Johnson leading stratiform type system.† Rain cooled air reinforces the boundary as the initial cell moves away from it only to be replaced by a new cell developing along it.† The rain cooled air on the east side or northeast side of the boundary stabilized the air mass east of the boundary but moisture unstable air remains along its southwestern flank.† The new cells forming along the southwest flank means that the propagation due to new cell formation is opposing the movement of the system due to advection.† Therefore,† resultant MCS moves slower than the mean flow which helps prolong the period that high rainfall rates will be affecting the area.†

 

The likelihood of multiple cell training across an area increases as new cells generate which also tends to increase the rainfall efficiency of the system by increasing the relative humidity and decreasing the negative effects of† entrainment.† Itís important when you think that the pattern favors a Maddox type heavy rainfall event to still evaluate the pattern using an ingredients based methodology.† Try to anticipate which is the most likely Parker and Johnson arch-type is the most likely to develop.† For example,† is new convection likely to develop upstream or downstream from the initial convection that develops.†

 

An example that fits the Maddox et al. archtype and using climatological thickness to forecast where an MCS might form

Now letís try to apply what youíve learned from studying about the Maddox arch-types.† However,† Weíll also try to apply climatological thickness to helping solve where the axis of heavy rain will fall.† During the 1960ís one of the forecasters conducted a study to try to find out the average value of 1000-500-hPa thickness values that 1.00 inch or more rainfall was observed in a 12 hour period during the various months of the year (Bohl and Junker,† 1987).† HPC have used the climatological thickness to try to help forecast where convection might develop.†† Climatological thickness works best during the summer when dynamics are relatively weak. Letís try to use the climatological thickness to estimate the most likely region where heavy precipitation might develop using the fields shown above.† Note that a shortwave is approaching the 500 hPa ridge axis.† Such a 500 hPa pattern is a common one for Maddox frontal and meso-high events.†† The graph below shows the climatological thickness for the northern plains and portions of the upper Midwest.†† The mean thickness for region 1 using the chart below is 575.† However,††† the median is 576 and the mode 579 for the region suggesting that the axis of heavy rain will† occur in northern Nebraska extreme northwestern Iowa.†

 

 

MSL and 1000-500 thickness forecast

582

579

576

585

573

A)

Click A to go to a with the climatolgical thickness for various regions of the country.

588

500-hPA height and vorticity forecast

00 UTC 17 July

00 UTC 17 July

NAM forecast of precipitable water and 850 hPa wind

Valid 00 UTC 17 July

Valid 03 UTC 17 July

1.25

1.25

1.50

1.50

Forecasters at HPC have found that the amount of available moisture in the atmosphere plays a strong role in determining the whether heaviest convection will occur along or north of the front to whether outflow boundaries from the initial convection will help focus convection south of the front. The forecasters also have noted that when the mean relative humidity reaches 70% or more the lifting is present, convection usually develops.† This relationship is the basis for using the saturation thickness table.† If for example the average PW values south of the front are about 1.15 to 1.25 inches, then saturation would most likely occur right along the front as represented on the bottom figure on the right hand side.† If on the other hand, the PW values were lower, letís say 0.80Ē, then for hypothetical case below, then north of the boundary.†

From Funk 1999

If you were using the saturated thickness concept to predict the axis of heaviest rainfall for the 17 July case,† the new forecast axis would be:

A).

north or northeast of the location found using the climatological thickness.

B).

Almost identical to the one found using climatological thickness

C).

South or southeast of the location suggested by climatological thickness.†

Please click on the correct answer, A, B, or C.