In an article from the Medium Wave circle called"The Wetter The Better"
by Andy Sennitt ,Steve Whitt and David Thorpe of the U.K. write about
their observations on the enhancement of medium wave signals by wet
weather.This was discussed by Steve on the Jan 28th 1999 edition of
the Media Network radio show.
In this same article David Thorpe says""I can confirm your suspicions.
Wet weather does indeed have an effect on
ground conductivity. In the past measurements have been made over a
50km path from a London mf tx site,
and the field strength of the received signal, measured at the same
location, has been more than 2 db uV
greater during wet weather conditions".
From Radioelectronics.com an article on ground wave signal
propagation says:"


Effect of frequency

As the wavefront of the ground wave travels along the Earth's surface
it is attenuated. The degree of attenuation is dependent upon a
variety of factors. Frequency of the radio signal is one of the major
determining factor as losses rise with increasing frequency. As a
result it makes this form of propagation impracticable above the
bottom end of the HF portion of the spectrum (3 MHz). Typically a
signal at 3.0 MHz will suffer an attenuation that may be in the region
of 20 to 60 dB more than one at 0.5 MHz dependent upon a variety of
factors in the signal path including the distance. In view of this it
can be seen why even high power HF radio broadcast stations may only
be audible for a few miles from the transmitting site via the ground
wave."
I would conclude from these articles that wet weather, especially
flooding, does indeed enhance ground wave propagation.Since there is
an attenuation at higher frequencies in the medium wave band;the high
end of the band might benefit the most(since signals usually do not
travel so
far) through enhanced ground wave under these wet conditions.